md: update MAINTAINERS info

I have been reviewing patches for md in the past few months. Mark me
as the MD maintainer, as I have effectively been filling that role.

Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>