mm,hwpoison: unify THP handling for hard and soft offline

mainline inclusion
from linux-v5.10-rc1
commit 694bf0b0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4LE22


CVE: NA

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Place the THP's page handling in a helper and use it from both hard and
soft-offline machinery, so we get rid of some duplicated code.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>