cpuset: introduce CS_ONLINE

Add CS_ONLINE which is set from css_online() and cleared from
css_offline().  This will enable using generic cgroup iterator while
allowing decoupling cpuset from cgroup internal locking.

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

diff --cc kernel/cpuset.c
Simple merge