cgroup: Fix kabi broken by files_cgroup introduced

hulk inclusion
category: feature/cgroups
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=7


CVE: NA

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The patch adds ifndef(__GENKSYMS__) into cgroup_subsys.h, and this
change is more nasty than it looks. It hides the fact that that we change
the layout of "struct cgroup" and "struct css_set", they both have the
subsys[CGROUP_SUBSYS_COUNT] member. I hope this is fine, the modular code
has no reasons to access the private members after ->subsys[], and the
helpers like cgroup_sane_behavior() shouldn't be used by external modules.

The patch also fixes the following compile warning caused by the fix kabi
broken.

Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>