fellow_cache: Fix fellow_cache_seglist_init() size parameter semantics

before this change, it was taken as excluding struct fellow_cache_seglist,
and was correctly used at the call sites changed with this commit
(where (size - sizeof *fcsl) was used as the argument).

However, for the calls from
- fellow_busy_obj_alloc()
- fellow_cache_obj_get()

there was a mismatch with the caller's size value.

src/fellow_cache.c

@@ -2026,7 +2026,7 @@ fellow_cache_seglist_associate(
 }
 
 static struct fellow_cache_seglist *
-fellow_cache_seglist_init(void *ptr, size_t space, struct fellow_cache_obj *fco)
+fellow_cache_seglist_init(void *ptr, size_t sz, struct fellow_cache_obj *fco)
 {
 	struct fellow_cache_seglist *fcsl = ptr;
 	size_t lsegs;
@@ -2034,9 +2034,11 @@ fellow_cache_seglist_init(void *ptr, size_t space, struct fellow_cache_obj *fco)
 
 	AN(ptr);
 	assert(PAOK(ptr));
-	assert(space >= sizeof *fcsl);
+	assert(sz > sizeof *fcsl);
 	INIT_OBJ(fcsl, FELLOW_CACHE_SEGLIST_MAGIC);
-	lsegs = space / sizeof *fcsl->segs;
+	sz -= sizeof *fcsl;
+	lsegs = sz / sizeof *fcsl->segs;
+	assert(lsegs > 0);
 	assert(lsegs <= UINT16_MAX);
 	fcsl->lsegs = (uint16_t)lsegs;
 	for (u = 0; u < fcsl->lsegs; u++)
@@ -2236,8 +2238,7 @@ fellow_cache_seglists_load(struct worker *wrk, void *priv)
 		}
 
 		ofcsl = fcsl;
-		fcsl = fellow_cache_seglist_init(fcsl_mem.ptr,
-		    fcsl_mem.size - sizeof *fcsl, fco);
+		fcsl = fellow_cache_seglist_init(fcsl_mem.ptr, fcsl_mem.size, fco);
 		fcsl->next = fcsl_pending;
 		fcsl->fdsl_sz = fdsl_mem.size;
 		fcsl->fcsl_sz = fcsl_mem.size;
@@ -2889,8 +2890,7 @@ fellow_disk_seglist_alloc(struct fellow_busy *fbo,
 	fdsl = fellow_disk_seglist_init(fdsl_mem.ptr, ldsegs, fht);
 
 	AN(fcsl_mem.ptr);
-	fcsl = fellow_cache_seglist_init(fcsl_mem.ptr,
-	    fcsl_mem.size - sizeof *fcsl, fbo->fco);
+	fcsl = fellow_cache_seglist_init(fcsl_mem.ptr, fcsl_mem.size, fbo->fco);
 
 	fellow_cache_seglist_associate(fcsl, fdsl, FCS_USABLE);
 
@@ -5917,8 +5917,7 @@ struct objcore **ocp, uintptr_t priv2, unsigned crit)
 			err = FC_ERRSTR("first disk seglist fcsl alloc failed");
 			goto err;
 		}
-		fco->fcsl = fellow_cache_seglist_init(fcsl_mem.ptr,
-		    fcsl_mem.size - sizeof *fco->fcsl, fco);
+		fco->fcsl = fellow_cache_seglist_init(fcsl_mem.ptr, fcsl_mem.size, fco);
 		AN(fco->fcsl);
 		fco->fcsl->fcsl_sz = fcsl_mem.size;
 	}