mm: memcontrol: add struct mem_cgroup_extension

hulk inclusion
category: bugfix
bugzilla: 47240
CVE: NA

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Change struct mem_cgroup directly will causes the kabi broken.
So add a new struct mem_cgroup_extension for new variables.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>