mm: Add reliable_nr_page for accounting reliable memory

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


CVE: NA

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Adding an variable in mm_struct for accouting the amount of reliable memory
allocated by the reliable user tasks.

Signed-off-by: Peng Wu <wupeng58@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>