cpus: Introduce cpu_list_generation_id

cpus: Introduce cpu_list_generation_id

Introduce cpu_list_generation_id to track cpu list generation so
that cpu hotplug/unplug can be detected during measurement of
dirty page rate.

cpu_list_generation_id could be used to detect changes of cpu
list, which is prepared for dirty page rate measurement.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <06e1f1362b2501a471dce796abb065b04f320fa5.1656177590.git.huangy81@chinatelecom.cn>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>