mm: hugetlb: move hugeltb sysctls to its own file

This moves all hugetlb sysctls to its own file, also kill an
useless hugetlb_treat_movable_handler() defination.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>