mm/migration: fix possible do_pages_stat_array racing with memory offline

When follow_page peeks a page, the page could be migrated and then be
offlined while it's still being used by the do_pages_stat_array().  Use
FOLL_GET to hold the page refcnt to fix this potential race.

Link: https://lkml.kernel.org/r/20220318111709.60311-12-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>