cgroup: check if cgroup root is alive in cgroupstats_show()

euler inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8QLND


CVE: N/A

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If a cgroup root is dying, show its hierarchy_id and num_cgroups
as 0.

Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: chenridong <chenridong@huawei.com>