mm: mempool: fix wake-up edge case bug for zero-minimum pools

The mempool wake-up path has a edge case bug that affects pools created
with min_nr=0.  When a thread blocks waiting for memory from an empty pool
(curr_nr == 0), subsequent mempool_free() calls fail to wake the waiting
thread because the condition "curr_nr < min_nr" evaluates to "0 < 0" which
is false, this can cause threads to sleep indefinitely according to the
code logic.

There is at least 2 places where the mempool created with min_nr=0:

1. lib/btree.c:191: mempool_create(0, btree_alloc, btree_free, NULL)
2. drivers/md/dm-verity-fec.c:791:
 mempool_init_slab_pool(&f->extra_pool, 0, f->cache)

Add an explicit check in mempool_free() to handle the min_nr=0 case: when
the pool has zero minimum reserves, is currently empty, and has active
waiters, allocate the element then wake up the sleeper.

Link: https://lkml.kernel.org/r/f28a81ba-615c-481e-86fb-c0bf4115ec89@suse.com


Signed-off-by: Yadan Fan <ydfan@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>