Btrfs: fix memory leak in do_walk_down

BugLink: http://bugs.launchpad.net/bugs/1654602

commit a958eab0 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/btrfs/extent-tree.c

@@ -8488,6 +8488,7 @@ 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;
 	}