cgroup/files: support boot parameter to control if disable files cgroup

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8ND8I



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When files cgroup is enabled, it's will leads syscall performance
regression in UnixBench. Add a helper files_cgroup_enabled() and
use it to control if use files cgroup, wen can use cgroup_disable=files
in cmdline to disable files cgroup.

syscall of UnixBench (large is better)
enable files cgroup:            2868.5
disable files cgroup:           3177.0
disable config of files cgroup: 3186.5

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Chen Ridong <chenridong@huawei.com>