MAINTAINERS: move myself from ceph "Maintainer" to "Reviewer"

Xiubo has graciously volunteered to take over for me as the Linux cephfs
client maintainer. Make it official by changing myself to be a
"Reviewer" for libceph and ceph.

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