The exp thread is also critical. It must not starve, otherwise lru
starves also because of hitting OC_F_DYING objects.

Unfortunately, this brings us above 4K, so we fill up with io structs

This way we have a better chance to call the actual return at a more
suitable time, for example outside a lock.

Ref #53

Fixes #52

Closes #50

The fellow_e00029.vtc canary fell over on Ubuntu 20.04 with kernel 5.15

This fixes two bugs:

The most relavant was that we did not increase buddy->wait_pri if the
new priority was higher, which could lead to all kinds of weird effects,
for example:

- starvation/lock up
- LRU removing all of the cache

because there were requests waiting, but buddy_wait_work() did not see
them.

The second bug is that we also called buddy_wait_work() if there
actually was no change (the critical region could find state ==
IW_SIGNALLED).

Also, for the new assertions, we need to set the proper wait_pri for the
case that the priority is lowered.

we did not lower buddy->wait_pri if the cancel resulted in an empty
priority list. This was no problem, but the stricter assertions from the
previous commit would trigger, because they now require that the
priority list for wait_pri is filled.

For clarity and to not overlook it, pools should have a proper default
priority. The fill callback can still change it.