cgroup: more naming cleanups

Constantly use @cset for css_set variables and use @cgrp as cgroup
variables.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --cc include/linux/cgroup.h
Simple merge

diff --cc kernel/cgroup.c
Simple merge

diff --cc kernel/cpuset.c
Simple merge