Add a new "r_ended" field to struct ceph_mds_request and use that to
maintain the average latency of MDS requests.

URL: https://tracker.ceph.com/issues/43215Signed-off-by: NXiubo Li <xiubli@redhat.com>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>