ceph: return ETIMEDOUT errno to userland when request timed out

req->r_timeout is only used during mounting, so this error will
be more accurate.

URL: https://tracker.ceph.com/issues/44215


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