sched: Clean up ttwu() rq locking

Since set_task_clock() doesn't rely on rq->clock anymore we can simplyfy
the mess in ttwu().

Optimize things a bit by not fiddling with the IRQ state there.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>