cgroup: Fix incorrect css_set_rwsem reference in comment

Since commit f0d9a5f1 ("cgroup: make css_set_rwsem a spinlock
and rename it to css_set_lock"), css_set_rwsem has been replaced by
css_set_lock. That commit, however, missed the css_set_rwsem reference
in include/linux/cgroup-defs.h. Fix that by changing it to css_set_lock
as well.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>