mm/dynamic_hugetlb: add PG_pool flag to mark pages from dhugetlb_pool

hulk inclusion
category: feature
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4QSHG


CVE: NA

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PG_pool is used to identify whether a page is belonging to a hugetlb_pool.

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