Revert "sched: Remove get_online_cpus() usage"

This reverts commit 5b2be7076053a97f72e9370a5869b76ad9c547b5.
Revert "sched: Remove extra put_online_cpus() inside sched_setaffinity()"

This reverts commit eddc1f8869d833e4848081fa70369f18e6d9b611.
Revert "sched: Fix endless sync_sched/rcu() loop inside _cpu_down()"

This reverts commit f427401d9bccac4fde9eb0656398bb8e1d273ee9.
Revert "sched: Remove get_online_cpus() usage"

This reverts commit e08f955275df58eebec53b1fdef5f5cb66893ee0.