MAINTAINERS: add Muchun as co-maintainer for HugeTLB

I have been focusing on mm for the past two years.  e.g.  developing,
fixing bugs, reviewing related to HugeTLB system.  I would like to help
Mike and other people working on HugeTLB by reviewing their work.

When I first introduced the vmemmmap reduction, I forgot to update
MAINTAINERS file.  Let's update it as well.  And rename "HUGETLB
FILESYSTEM" to "HUGETLB SUBSYSTEM" since some files are not only related
to filesystem but also memory management (the name of FILESYSTEM cannot
cover this area).

Link: https://lkml.kernel.org/r/20220521074103.79468-1-songmuchun@bytedance.com


Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>