sched, hotplug: cpu-hotplug vs. sched synchronization

Hi,

RFC, because I obviously might have missed something important and if not, there can be better ways to address the issues (a few alternatives included).

[1/2] safe use of rq->migration_thread and find_busiest_queue()

[2/2] ensure a task is on the valid cpu after set_cpus_allowed_ptr()

--Dmitry