mm/memcg: introduce mem_cgroup_scan_cgroups to scan all memcgs

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8S9BY


CVE: NA

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Introduce mem_cgroup_scan_cgroups to iterate over memcgs of a memory
cgroup hierarchy.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>