mm: Export static key mem_reliable

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


CVE: NA

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Static key mem_reliable is used to check wheater memory reliable's status
in kernel's inline functions. These inline function rely on this but dirver
can not use because this symbol is not exported.
To slove this problem, export this symbol to make prepration for driver to
use memory reliable's inline function.

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