bcache: Fix a minor memory leak on device teardown

Reported-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>

drivers/md/bcache/super.c

@@ -64,9 +64,11 @@ struct workqueue_struct *bcache_wq;
 
 static void bio_split_pool_free(struct bio_split_pool *p)
 {
+	if (p->bio_split_hook)
+		mempool_destroy(p->bio_split_hook);
+
 	if (p->bio_split)
 		bioset_free(p->bio_split);
-
 }
 
 static int bio_split_pool_init(struct bio_split_pool *p)