memcg: Export memcg.{min/low/high} from cgroupv2 to cgroupv1

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8JVN0



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Export memcg.min and memcg.low from cgroupv2 to cgroupv1, in order to
reduce the negtive impact between cgroups when the system memory is
insufficient.

Export memory.high from cgroupv2 to cgroupv1. Therefore, when the usage
of the memcg is larger than memory.high, some pages will be reclaimed
before return to userland, which will throttle the process.

Only export memory.{min/low/high} numbers in mem_cgroup_legacy_files and
declare related functions in front of mem_cgroup_legacy_files.
There is no need to other changes.

Signed-off-by: Lu Jialin <lujialin4@huawei.com>