mm/dynamic_hugetlb: use pfn to traverse subpages

hulk inclusion
category: bugfix
bugzilla: 46904 https://gitee.com/openeuler/kernel/issues/I4Y0XO



--------------------------------

For 1G huge pages, the struct page of each subpages may be discontinuous, but
pfn must be continuous, so it's better to traverse subpages using pfn rathan
than struct page.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>