Fixes #26 specifically, we are still missing similar handling in other
places.

2d468a6f gave the clue:

When fellow_cache_obj_lru_touch() raced fellow_cache_lru_work(),
we would put the fco's fcs on the lru list twice: once from
_touch and then from _lru_work().

Fixes #25

See next commit for fix

This reverts commit bf1e9e71.

Conflicts:
	src/fellow_cache.c

Conflicts:
	src/fellow_cache.c

Motivated by #25, which looks like a self-induced deadlock
in fellow_cache_lru_work()

I suspected we could run into an infinite loop here. Suppose there is
only one fco on the LRU and we fail to mutate it, then it was
re-inserted again and again.

Related to #25, but I am not sure yet if it is the root cause

Avoid memory LRU racing disk LRU.

Disk LRU uses the varnish-cache LRU facility, which works by setting
the OC_F_DYING and gaining one reference, resulting in two references.

One is lost again by the thread initiating the LRU nuke, the other by
the EXP thread. Between the two events, the refcnt is one again, thus
stvfe_mutate could race.

I believe this fixes #23 and #24. If not, please reopen

Tests #22

the test case from the next commit exposed a deadlock because the
only object in the test would consume all memory und could not
get LRUd because fellow_cache_async_write_complete() would hold
the fco mtx during log submission.

UINT16_MAX would be just over a disk block, and allocating another
3.93k for a single additional segment does not make sense.

	(gdb) p /x sizeof(struct fellow_disk_seg) * 65534 +
		sizeof(struct fellow_disk_seglist)
	$7 = 0x37ffd0

Also fix a potential overflow, where a uint16_t was incremented after
clamping to UINT16_MAX.

Tests #22

Fixes another case of #22

Fixes one case for #22

fellow_cache_lru_chg() already calls fellow_cache_lru_chgbatch_apply()
when the remove array is full.

Fix a regression from 44d788bf:

While we do want to reduce the critical region holding the lru mtx,
we can not release the fco mtx before we have completed the trans-
action on it with respect to LRU state.

Because we might need to un-do the LRU removal of the FCO, we
need to keep the mtx held until we know.

Otherwise another thread can race us and change the state under
our feet.

In this case, we raced fellow_cache_obj_delete():

 #9  0x00007f2711972fd6 in __GI___assert_fail (
     assertion=assertion@entry=0x7f27116f3ec1 "(fcs->fcs_onlru) != 0",
     file=file@entry=0x7f27116f31f8 "fellow_cache.c", line=line@entry=3145,
     function=function@entry=0x7f27116f6b50 <__PRETTY_FUNCTION__.13829> "fellow_cache_lru_work") at assert.c:101
 #10 0x00007f27116bd1db in fellow_cache_lru_work (wrk=wrk@entry=0x7edb0a8135d0, lru=lru@entry=0x7edb4421eb10)
     at fellow_cache.c:3145
 #11 0x00007f27116bd7c7 in fellow_cache_lru_thread (wrk=0x7edb0a8135d0, priv=0x7edb4421eb10)
     at fellow_cache.c:3322
 #12 0x000056544bcc06cb in wrk_bgthread (arg=0x7edb3a6e0900) at cache/cache_wrk.c:104
 #13 0x00007f2711b39609 in start_thread (arg=<optimized out>) at pthread_create.c:477
 #14 0x00007f2711a5e133 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

(gdb) p *fcs
$1 = {magic = 25208, state = FCO_INCORE, fcs_onlru = 0, fco_infdb = 0, lcb_add = 0, lcb_remove = 0,
  fco_lru_mutate = 0, refcnt = 0, lru_list = {vtqe_next = 0x7eebee7a00a0, vtqe_prev = 0x7edb4421eb48},
  fco = 0x7ef07cf3c000, disk_seg = 0x7ef1cd8e8008, alloc = {ptr = 0x7ef1cd8e8000, size = 4096}, len = 0}
(gdb) p *fcs->fco
$2 = {magic = 2206029151, logstate = FCOL_DELETED, lru = 0x7edb4421eb10, fco_mem = {ptr = 0x7ef07cf3c000,
    bits = 13 '\r', magic = 4294193151}, mtx = pthread_mutex_t = {Type = Normal,
    Status = Acquired, possibly with waiters, Owner ID = 543234, Robust = No, Shared = No, Protocol = None},
  cond = pthread_cond_t = {Threads known to still execute a wait function = 0, Clock ID = CLOCK_REALTIME,
    Shared = No}, oc = 0x7ed3c82b0b00, fdb = {fdb = 2493649440769}, fdb_entry = {rbe_link = {0x7eebec72c000,
      0x0, 0x0}}, fdo_fcs = {magic = 25208, state = FCO_INCORE, fcs_onlru = 0, fco_infdb = 0, lcb_add = 0,
    lcb_remove = 0, fco_lru_mutate = 0, refcnt = 0, lru_list = {vtqe_next = 0x7eebee7a00a0,
      vtqe_prev = 0x7edb4421eb48}, fco = 0x7ef07cf3c000, disk_seg = 0x7ef1cd8e8008, alloc = {
      ptr = 0x7ef1cd8e8000, size = 4096}, len = 0}, aa_esidata_seg = {magic = 25208, state = FCS_USABLE,
    fcs_onlru = 0, fco_infdb = 0, lcb_add = 0, lcb_remove = 0, fco_lru_mutate = 0, refcnt = 0, lru_list = {
      vtqe_next = 0x0, vtqe_prev = 0x0}, fco = 0x7ef07cf3c000, disk_seg = 0x7ef1cd8e80f0, alloc = {ptr = 0x0,
      size = 0}, len = 0}, seglist = {magic = 3403082203, lsegs = 122, fdsl = 0x7ef1cd8e8178, fdsl_sz = 0,
    fcsl_sz = 0, next = 0x0, segs = 0x7ef07cf3c148}}

racing thread:

 Thread 3478 (Thread 0x7f2705d84700 (LWP 543234)):
 #0  __lll_lock_wait (futex=futex@entry=0x7edb4421eb20, private=0) at lowlevellock.c:52
 #1  0x00007f2711b3c0a3 in __GI___pthread_mutex_lock (mutex=mutex@entry=0x7edb4421eb20) at ../nptl/pthread_mutex_lock.c:80
 #2  0x00007f27116ab718 in fellow_cache_lru_chgbatch_apply (lcb=lcb@entry=0x7f2705d813f0) at fellow_cache.c:1104
 #3  0x00007f27116bf7b0 in fellow_cache_obj_delete (fc=0x7f27112ed000, fco=<optimized out>, fco@entry=0x7ef07cf3c000, hash=hash@entry=0x7ed6fb0c69b0 "b5*\371\064\062j\362\212Ze礤(X0լ\266\216JL&\231\223\302\031\315\365\277\n") at fellow_cache.c:4808
 #4  0x00007f271167eec2 in sfemem_free (wrk=wrk@entry=0x7f2705d825d0, memoc=memoc@entry=0x7ed3c82b0b00) at fellow_storage.c:543
 #5  0x00007f271167f365 in sfemem_objfree (wrk=0x7f2705d825d0, memoc=0x7ed3c82b0b00) at fellow_storage.c:577
 #6  0x000056544bc964aa in ObjFreeObj (wrk=wrk@entry=0x7f2705d825d0, oc=0x7ed3c82b0b00) at cache/cache_obj.c:412
 #7  0x000056544bc8ce8f in HSH_DerefObjCore (wrk=0x7f2705d825d0, ocp=ocp@entry=0x7f2705d82360, rushmax=rushmax@entry=0) at cache/cache_hash.c:1059
 #8  0x000056544bc81530 in exp_expire (now=1691019717.3146894, ep=0x7f2711246280) at cache/cache_expire.c:360