MAINTAINERS: take over for Zheng as CephFS kernel client maintainer

Zheng wants to be able to spend more time working on the MDS, so I've
volunteered to take over for him as the CephFS kernel client maintainer.

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