memcg: Fix inconsistent oom event behavior for OOM_MEMCG_KILL

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4X0YD?from=project-issue


CVE: NA

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Since memory.event is fully supported in cgroupv1, the problem of inconsistent
oom event behavior for OOM_MEMCG_KILL occurs again.
We fix the problem by add a new condition to support the event adding
continue. Therefore, there are two condition:
1) memcg is not root memcg;
2) the memcg is root memcg and the event is OOM_MEMCG_KILL of cgroupv1

Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>