Btrfs: fix memory leak in do_walk_down

The extent buffer 'next' needs to be free'd conditionally.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -8891,6 +8891,7 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 				       &wc->flags[level - 1]);
 	if (ret < 0) {
 		btrfs_tree_unlock(next);
+		free_extent_buffer(next);
 		return ret;
 	}