dm cache: fix leaking of deferred bio prison cells

There were two cases where dm_cell_visit_release() was being called,
which removes the cell from the prison's rbtree, but the callers didn't
also return the cell to the mempool.  Fix this by having them call
free_prison_cell().

This leak manifested as the 'kmalloc-96' slab growing until OOM.

Fixes: 651f5fa2a3 ("dm cache: defer whole cells")
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.1+

drivers/md/dm-cache-target.c

@@ -1730,6 +1730,8 @@ static void remap_cell_to_origin_clear_discard(struct cache *cache,
 		remap_to_origin(cache, bio);
 		issue(cache, bio);
 	}
+
+	free_prison_cell(cache, cell);
 }
 
 static void remap_cell_to_cache_dirty(struct cache *cache, struct dm_bio_prison_cell *cell,
@@ -1764,6 +1766,8 @@ static void remap_cell_to_cache_dirty(struct cache *cache, struct dm_bio_prison_
 		remap_to_cache(cache, bio, cblock);
 		issue(cache, bio);
 	}
+
+	free_prison_cell(cache, cell);
 }
 
 /*----------------------------------------------------------------*/