mbcache: make sure c_entry_count is not decremented past zero

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
CC: Eric Biggers <ebiggers@google.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Jan Kara <jack@suse.cz>

fs/mbcache.c

@@ -239,7 +239,9 @@ void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 			spin_lock(&cache->c_list_lock);
 			if (!list_empty(&entry->e_list)) {
 				list_del_init(&entry->e_list);
-				cache->c_entry_count--;
+				if (!WARN_ONCE(cache->c_entry_count == 0,
+		"mbcache: attempt to decrement c_entry_count past zero"))
+					cache->c_entry_count--;
 				atomic_dec(&entry->e_refcnt);
 			}
 			spin_unlock(&cache->c_list_lock);