Revert "Revert "sched: Remove get_online_cpus() usage""

This reverts commit 3706e43478e9e21201678568d3fddef3f78d5536.
Revert "sched: Remove extra put_online_cpus() inside sched_setaffinity()"

This reverts commit 108b60d9eb4252bb2829682448ea31c0999a03d6.
Revert "sched: Remove get_online_cpus() usage"

This reverts commit bc1e9ac00a7729d863999eb6297c64acf1082aef.

Conflicts:
	kernel/cpu.c
Revert "sched: Fix endless sync_sched/rcu() loop inside _cpu_down()"

This reverts commit 66114a5914e568269e4ebec9ce3b35518e264fe0.

Conflicts:
	kernel/cpu.c