dm thin: reinstate missing mempool_free in cell_release_singleton

Fix a significant memory leak inadvertently introduced during
simplification of cell_release_singleton() in commit
6f94a4c4 ("dm thin: fix stacked bi_next
usage").

A cell's hlist_del() must be accompanied by a mempool_free().
Use __cell_release() to do this, like before.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-thin.c

@@ -279,8 +279,10 @@ static void __cell_release(struct cell *cell, struct bio_list *inmates)
 
 	hlist_del(&cell->list);
 
-	bio_list_add(inmates, cell->holder);
-	bio_list_merge(inmates, &cell->bios);
+	if (inmates) {
+		bio_list_add(inmates, cell->holder);
+		bio_list_merge(inmates, &cell->bios);
+	}
 
 	mempool_free(cell, prison->cell_pool);
 }
@@ -303,9 +305,10 @@ static void cell_release(struct cell *cell, struct bio_list *bios)
  */
 static void __cell_release_singleton(struct cell *cell, struct bio *bio)
 {
-	hlist_del(&cell->list);
 	BUG_ON(cell->holder != bio);
 	BUG_ON(!bio_list_empty(&cell->bios));
+
+	__cell_release(cell, NULL);
 }
 
 static void cell_release_singleton(struct cell *cell, struct bio *bio)