mm/dynamic_hugetlb: initialize subpages before merging

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



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Patch ("hugetlb: address ref count racing in prep_compound_gigantic_page") add
a check of ref count in prep_compound_gigantic_page. We will call this function
in dynamic hugetlb feature too, so we should initialize subpages before calling
prep_compound_gigantic_page to satisfy the change.
Further, the input of prep_compound_gigantic_page should be a group of pages
rather than compound page, so clear the properties related to compound 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>