ceph: use timespec64 for r_stamp

The ceph_mds_request stamp still uses the deprecated timespec structure,
this converts it over as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>