In both preempt_schedule in sched.c and resume_kernel in entry.S, it is
possible to return with need_resched set and thus a pending preemption
but not service that preemption for some time.

Consider:

	- return from schedule() to preempt_schedule
	- interrupt occurs, sets need_resched
	- we cannot preempt since preempt_count = PREEMPT_ACTIVE
	- back in preempt_schedule, set preempt_count = 0

Now we again can preempt, but we will not.  Instead we return and
continue executing.  On the next interrupt, we will redo the whole
fiasco which is a waste since we could of reentered schedule while we
were there.  Worse, if we acquire a lock before the next interrupt we
can potentially delay the pending reschedule a very long time.  This is
not acceptable.

The solution is to check for and loop on need_resched on resume_kernel
and preempt_schedule like schedule itself does.