sched/hotplug: Catch straggler tasks on about-to-die CPUs

Only KTHREAD_IS_PER_CPU tasks should now be enqueued on an (dying)
online && !active CPU. As the about-to-die CPU goes further down its
hotplug steps, said tasks must be migrated or parked by their respective
owners. Failure to comply will make sched_cpu_dying() very unhappy.

Warn if any unexpected task gets enqueued beyond sched_cpu_wait_empty().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>