Revert "sched: Remove get_online_cpus() usage"

This reverts commit c93b0d8eb0d464f7a379a3b643b44a1af9a2962f.
Revert "sched: Remove extra put_online_cpus() inside sched_setaffinity()"

This reverts commit d3a61784a337d4599ab30c69871db15d8856e200.
Revert "sched: Fix endless sync_sched/rcu() loop inside _cpu_down()"

This reverts commit f6250dab1cc28d4a0e057caaf8a90956ff56fd6f.
Revert "sched: Remove get_online_cpus() usage"

This reverts commit 4dd57e187817fcb798b5569c32cbff22fbffda5b.