Btrfs: avoid positive number with ERR_PTR

inode_ref_info() returns 1 when the element wasn't found and < 0 on error,
just like btrfs_search_slot(). In iref_to_path() it's an error when the
inode ref can't be found, thus we return ERR_PTR(ret) in that case. In order
to avoid ERR_PTR(1), we now set ret to -ENOENT in that case.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>