ceph: use ceph_mdsc_build_path instead of clone_dentry_name

While it may be slightly more efficient, it's probably not worthwhile to
optimize for the case that clone_dentry_name handles. We can get the
same result by just calling ceph_mdsc_build_path when the parent isn't
locked, with less code duplication.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>