mds: only kick head on snap rdlock if auth
mds: only kick head on snap rdlock if auth

- If we are non-auth, stick with the snap, and the auth will do the
inference.
- If we are auth, the head had better exist, because our lock is
pinned in an unreadable state for some reason.  Assert as much.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph