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.