cls_cgroup: clean up for cgroup part

- It's better to use container_of() instead of casting cgroup_subsys_state *
  to cgroup_cls_state *.
- Add helper function task_cls_state().
- Rename net_cls_state() to cgrp_cls_state().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>