cgroup: update comment about dropping cgroup kn refs

the cgroup is actually freed in css_free_rwork_fn() now
the ref count of the cgroup's kernfs_node is also dropped there
so we need to update the corresponding comment in cgroup_mkdir()

Signed-off-by: Haorui He <mail@hehaorui.com>
Signed-off-by: Tejun Heo <tj@kernel.org>