srcu: use cpu_online() instead custom check

The current check via srcu_online is slightly racy because after looking
at srcu_online there could be an interrupt that interrupted us long
enough until the CPU we checked against went offline.
An alternative would be to hold the hotplug rwsem (so the CPUs don't
change their state) and then check based on cpu_online() if we queue it
on a specific CPU or not. queue_work_on() itself can handle if something
is enqueued on an offline CPU but a timer which is enqueued on an offline
CPU won't fire until the CPU is back online.

I am not sure if the removal in rcu_init() is okay or not. I assume that
SRCU won't enqueue a work item before SRCU is up and ready.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>