memcg: support ksm merge any mode per cgroup

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I72R0B


CVE: NA

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Add control file "memory.ksm" to enable ksm per cgroup.
Echo to 1 will set all tasks currently in the cgroup to ksm merge
any mode, which means ksm gets enabled for all vma's of a process.
Meanwhile echo to 0 will disable ksm for them and unmerge the
merged pages.
Cat the file will show the above state and ksm related profits
of this cgroup.

Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>