cgroup/cpuset: Fix obsolete comment in cpuset_css_offline()

Move the partition disablement comment from cpuset_css_offline()
to cpuset_css_killed() and reword it to remove the remnant of
sched.partition.

Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>