ceph: close holes in structs ceph_mds_session and ceph_mds_request

Move s_ref up to plug a 4 byte hole, which plugs another.
Move r_kref to shave 8 bytes off per request on x86_64.

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