Revert "sched: Fix endless sync_sched/rcu() loop inside _cpu_down()"

This reverts commit 75b9361c9b18d7e864d520c075c4a06de2343f87.
Revert "sched: Remove extra put_online_cpus() inside sched_setaffinity()"

This reverts commit e64bbe3abf738cab86a428e061366ef02f943738.
Revert "sched: Remove get_online_cpus() usage"

This reverts commit 414e6157dd85812e63a993680be6fac9dad13821.