cgroup: implement cgroup_from_id()

This will be used as a replacement for css_lookup().

There's a difference with cgroup id and css id. cgroup id starts with 0,
while css id starts with 1.

v4:
- also check if cggroup_mutex is held.
- make it an inline function.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>