mm/dynamic_hugetlb: improve the initialization of huge pages

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



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Referring to alloc_buddy_huge_page function, replace prep_compound_page
with prep_new_page which is more appropriate because it's the opposite of
free_pages_prepare.
And initialize page->mapping for huge pages as they are initialized in
free_huge_page too.

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