MAINTAINERS: add Xiubo Li as cephfs co-maintainer

Xiubo has been doing stellar kernel work lately, and has graciously
volunteered to help with maintainer duties. Add him on as co-maintainer
in for ceph.ko and libceph.ko.

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