Revert "sched: Remove get_online_cpus() usage"

This reverts commit d99575185a557c5db9a11dfe30cc4c9d0bda8f1b.
Revert "sched: Remove extra put_online_cpus() inside sched_setaffinity()"

This reverts commit 5019464044be7b0105e7bf6ff86632c39dacd8ef.
Revert "sched: Fix endless sync_sched/rcu() loop inside _cpu_down()"

This reverts commit b8f6a9228719e6c3ae432e51c0bdfdae64e60ee8.
Revert "sched: Remove get_online_cpus() usage"

This reverts commit 83f91f8e7557c1eedbd8c115cac47fb5437f6793.