ceph: return ceph_mdsc_do_request() errors from __get_parent()

Return the error returned by ceph_mdsc_do_request(). Otherwise,
r_target_inode ends up being NULL this ends up returning ENOENT
regardless of the error.

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>