sched: populate cpupool0 only after all cpus are up

sched: populate cpupool0 only after all cpus are up

Simplify cpupool initialization by populating cpupool0 with cpus only
after all cpus are up. This avoids having to call the cpu notifier
directly for cpu 0.

With that in place there is no need to create cpupool0 earlier, so
do that just before assigning the cpus. Initialize free cpus with all
online cpus at that time in order to be able to add the cpu notifier
late, too.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>