mm: mem_reliable: Update reliable page counter to zero if underflows

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I77BDW


CVE: NA

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Since reliable page counter is used for debug purpose only, There is no
real function problem by doing this.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Nanyong Sun <sunnanyong@huawei.com>