ceph: update i_requested_max_size only when sending cap msg to auth mds

Non-auth mds can't do anything to 'update max' cap message.

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