xen/sched: call cpu_disable_scheduler() via cpu notifier

xen/sched: call cpu_disable_scheduler() via cpu notifier

cpu_disable_scheduler() is being called from __cpu_disable() today.
There is no need to execute it on the cpu just being disabled, so use
the CPU_DEAD case of the cpu notifier chain. Moving the call out of
stop_machine() context is fine, as we just need to hold the domain RCU
lock and need the scheduler percpu data to be still allocated.

Add another hook for CPU_DOWN_PREPARE to bail out early in case
cpu_disable_scheduler() would fail. This will avoid crashes in rare
cases for cpu hotplug or suspend.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>