sched_ext: Implement cgroup sub-sched enabling and disabling

sched_ext: Implement cgroup sub-sched enabling and disabling

The preceding changes implemented the framework to support cgroup
sub-scheds and updated scheduling paths and kfuncs so that they have
minimal but working support for sub-scheds. However, actual sub-sched
enabling/disabling hasn't been implemented yet and all tasks stayed on
scx_root.

Implement cgroup sub-sched enabling and disabling to actually activate
sub-scheds:

- Both enable and disable operations bypass only the tasks in the subtree
  of the child being enabled or disabled to limit disruptions.

- When enabling, all candidate tasks are first initialized for the child
  sched. Once that succeeds, the tasks are exited for the parent and then
  switched over to the child. This adds a bit of complication but
  guarantees that child scheduler failures are always contained.

- Disabling works the same way in the other direction. However, when the
  parent may fail to initialize a task, disabling is propagated up to the
  parent. While this means that a parent sched fail due to a child sched
  event, the failure can only originate from the parent itself (its
  ops.init_task()). The only effect a malfunctioning child can have on the
  parent is attempting to move the tasks back to the parent.

After this change, although not all the necessary mechanisms are in place
yet, sub-scheds can take control of their tasks and schedule them.

v2: Fix missing scx_cgroup_unlock()/percpu_up_write() in abort path
    (Cheng-Yang Chou).

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