mm: zswap: fix pool refcount bug around shrink_worker()

When a zswap store fails due to the limit, it acquires a pool reference
and queues the shrinker.  When the shrinker runs, it drops the reference. 
However, there can be multiple store attempts before the shrinker wakes up
and runs once.  This results in reference leaks and eventual saturation
warnings for the pool refcount.

Fix this by dropping the reference again when the shrinker is already
queued.  This ensures one reference per shrinker run.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 45190f0 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Chris Mason <[email protected]>
Acked-by: Nhat Pham <[email protected]>
Cc: Vitaly Wool <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: <[email protected]>	[5.6+]
Signed-off-by: Andrew Morton <[email protected]>

Author: hnaz

mm/zswap.c

@@ -1383,8 +1383,8 @@ bool zswap_store(struct folio *folio)
 
 shrink:
 	pool = zswap_pool_last_get();
-	if (pool)
-		queue_work(shrink_wq, &pool->shrink_work);
+	if (pool && !queue_work(shrink_wq, &pool->shrink_work))
+		zswap_pool_put(pool);
 	goto reject;
 }