1) Fix the comments for the migration_thread.  A while back Ingo
   agreed they were exactly wrong, IIRC. 8).

2) Changed spin_lock_irqsave to spin_lock_irq, since it's in a
   kernel thread.

3) Don't repeat if the task has moved off the original CPU, just finish.
   This is because we are simply trying to push the task off this CPU:
   if it's already moved, great.  Currently we might theoretically move
   a task which is actually running on another CPU, which is v. bad.

4) Replace the __ffs(p->cpus_allowed) with any_online_cpu(), since
   that's what it's for, and __ffs() can give the wrong answer, eg. if
   there's no CPU 0.

5) Move the core functionality of migrate_task into a separate function,
   move_task_away, which I want for the hotplug CPU patch.