css task iteration will be updated to not leak cgroup internal locking
to iterator users.  In preparation, update css_set and task lists to
be in chronological order.

For tasks, as migration path is already using list_splice_tail_init(),
only cgroup_enable_task_cg_lists() and cgroup_post_fork() need
updating.  For css_sets, link_css_set() is the only place which needs
to be updated.

Signed-off-by: Tejun Heo <tj@kernel.org>