cpuset: relocate a misplaced comment

Comment for cpuset_css_offline() was on top of cpuset_css_free().
Move it.

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