memcg: support priority for oom

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZN3O


CVE: NA

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We first kill the process from the low priority memcg if OOM occurs.
If the process is not found, then fallback to normal handle.

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>