cgroup: Export cgroup.stat from cgroupv2 to cgroupv1

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8U6EM



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Export cgroup.stat feature from cgroupv2 to cgroupv1. Therefore, user
can get dying descendants cgroup number and descendants number in
cgroupv1.

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