sched/preempt: Fix up missed PREEMPT_NEED_RESCHED folding

With various drivers wanting to inject idle time; we get people
calling idle routines outside of the idle loop proper.

Therefore we need to be extra careful about not missing
TIF_NEED_RESCHED -> PREEMPT_NEED_RESCHED propagations.

While looking at this, I also realized there's a small window in the
existing idle loop where we can miss TIF_NEED_RESCHED; when it hits
right after the tif_need_resched() test at the end of the loop but
right before the need_resched() test at the start of the loop.

So move preempt_fold_need_resched() out of the loop where we're
guaranteed to have TIF_NEED_RESCHED set.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-x9jgh45oeayzajz2mjt0y7d6@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>