mm: Export mem_reliable_status() for checking memory reliable status

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4SK3S


CVE: NA

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Export the mem_reliable_status(), so it can be used by others to check
memory reliable's status.

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