ceph: don't reach into request header for readdir info

We already have a pointer to the argument struct in req->r_args. Use that
instead of groveling around in the ceph_mds_request_head.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>