Revert "sched: Remove get_online_cpus() usage"

This reverts commit 4a7f077c0e93b41b9f28b8f396de35d52e8bc1a7.

We are missing code from  kernel/stop_machine.c and it's cant be updated
till glocks will be update along with A LOT of FS code.

we need
https://github.com/torvalds/linux/commit/1be0bd77c5dd7c903f46abf52f9a3650face3c1d
and all next commits to fix the race and deadlock using glocks.

only then!
https://github.com/torvalds/linux/commit/6acce3ef84520537f8a09a12c9ddbe814a584dd2
not before.