This fixes the SMP runqueue locking bug when updating the wakers
priority.  It also includes:

 - only update the priority and do a requeueing if the sleep average has
   changed. (this does not happen for pure CPU hogs or pure interactive
   tasks, so no need to requeue/recalc-prio in that case.) [All the
   necessary values are available at that point already, so gcc should
   have an easy job making this branch really cheap.]

 - do not do a full task activation in the migration-thread path - that is
   supposed to be near-atomic anyway.

 - fix up comments

I solved the SMP locking bug by moving the requeueing outside of
try_to_wake_up().  It does not matter that the priority update is not
atomically done now, since the current process wont do anything
inbetween.  (well, it could get preempted in a preemptible kernel, but
even that wont do any harm.)