mm: Introduce reliable flag for user task

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4PM01


CVE: NA

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Adding reliable flag for user task. User task with reliable flag can
only alloc memory from mirrored region. PF_RELIABLE is added to represent
the task's reliable flag.

- For init task, which is regarded as special task which alloc memory
  from mirrored region.

- For normal user tasks, The reliable flag can be set via procfs interface
  shown as below and can be inherited via fork().

User can change a user task's reliable flag by

	$ echo [0/1] > /proc/<pid>/reliable

and check a user task's reliable flag by

	$ cat /proc/<pid>/reliable

Note, global init task's reliable file can not be accessed.

Signed-off-by: Peng Wu <wupeng58@huawei.com>
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>