ceph: always get rstat from auth mds

rstat is not tracked by capability. client can't know if rstat from
non-auth mds is uptodate or not.

Link: http://tracker.ceph.com/issues/23538


Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>