BUG/MEDIUM: pool: fix releasable pool calculation when overloaded

In 2.6-dev1, the method used to decide how many pool entries could be
released at once was revisited to support releases in batches. This was
done with commits 91a8e28f9 ("MINOR: pool: add a function to estimate
how many may be released at once") and 361e31e3f ("MEDIUM: pool: compute
the number of evictable entries once per pool").

The first commit takes care of the possible inconsistency between the
moment the allocated count and the used count are read, but unfortunately
fixed it the wrong way, by adjusting "used" to match "alloc" whenever it
was lower (i.e. almost always). This results in a nasty case which is that
as soon as the allocated value becomes higher than the estimated count of
needed entries, we end up returning pool->minavail, which causes very
small batches to be released, starting from commit 1513c5479 ("MEDIUM:
pools: release cached objects in batches").

The problem was further amplified in 2.9-dev3 with commit 7bf829ace
("MAJOR: pools: move the shared pool's free_list over multiple buckets")
because it now becomes possible for a thread to allocate from one bucket
and release into a few other different ones, causing an accumulation of
entries in that bucket.

The fix is trivial, simply adjust the alloc counter if the used one is
higher, before performing operations.

This must be backported to 2.6.

include/haproxy/pool.h

@@ -208,8 +208,8 @@ static inline uint pool_releasable(const struct pool_head *pool)
 
 	alloc = pool_allocated(pool);
 	used  = pool_used(pool);
-	if (used < alloc)
-		used = alloc;
+	if (used > alloc)
+		alloc = used;
 
 	needed_raw = pool_needed_avg(pool);
 	if (alloc < swrate_avg(needed_raw + needed_raw / 4, POOL_AVG_SAMPLES))