mm/dynamic_hugetlb: fix type error of pfn in __hpool_split_gigantic_page()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6XOIE


CVE: NA

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The type of pfn is int, which can result in truncation.
Change its type to unsigned long to fix the problem.

Fixes: eef7b4fd ("mm/dynamic_hugetlb: use pfn to traverse subpages")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>