sched_ext: Move disable machinery into scx_sched

sched_ext: Move disable machinery into scx_sched

Because disable can be triggered from any place and the scheduler cannot be
trusted, disable path uses an irq_work to bounce and a kthread_work which is
executed on an RT helper kthread to perform disable. These must be per
scheduler instance to guarantee forward progress. Move them into scx_sched.

- If an scx_sched is accessible, its helper kthread is always valid making
  the `helper` check in schedule_scx_disable_work() unnecessary. As the
  function becomes trivial after the removal of the test, inline it.

- scx_create_rt_helper() has only one user - creation of the disable helper
  kthread. Inline it into scx_alloc_and_add_sched().

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