mm: mem_reliable: Use zone_page_state to count free reliable pages

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6RKHX


CVE: NA

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commit ee97b610 ("mm: Count mirrored pages in buddy system") proposes
to use vmstat events (PGALLOC/PGFREE) to count free reliable pages.
However, this can be achieved by counting NR_FREE_PAGES for all
non-movable zones, which will have better compatibility. Therefore, replace
it.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>