sched_ext: Implement cgroup subtree iteration for scx_task_iter

sched_ext: Implement cgroup subtree iteration for scx_task_iter

For the planned cgroup sub-scheduler support, enable/disable operations are
going to be subtree specific and iterating all tasks in the system for those
operations can be unnecessarily expensive and disruptive.

cgroup already has mechanisms to perform subtree task iterations. Implement
cgroup subtree iteration for scx_task_iter:

- Add optional @cgrp to scx_task_iter_start() which enables cgroup subtree
  iteration.

- Make scx_task_iter use css_next_descendant_pre() and css_task_iter to
  iterate all tasks in the cgroup subtree.

- Update all existing callers to pass NULL to maintain current behavior.

The two iteration mechanisms are independent and duplicate. It's likely that
scx_tasks can be removed in favor of always using cgroup iteration if
CONFIG_SCHED_CLASS_EXT depends on CONFIG_CGROUPS.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>