btrfs: Check error of btrfs_iget in btrfs_search_path_in_tree_user

The patch introducing the ioctl was not the latest version at the time
of merging to the mainline and needs a fixup from this patch.

Fixes: ba637a252d30 ("btrfs: Check error of btrfs_iget() in btrfs_search_path_in_tree_user")
Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>