mm: add PG_pool in /proc/kpageflags

hulk inclusion
category: feature
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4RO84


CVE: NA

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Export PG_pool page flag in /proc/kpageflags.

	27. KPF_POOL		page is allocated from hpool.

By using this tool, we can easily count the number of pages allocated from
dynamic hugetlb pool by a process or a file.

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