Impact: micro-optimization

We can avoid the sched domain walk on try_to_wake_up() when we know
there are no groups.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1236603381.8389.455.camel@laptop>
Signed-off-by: NIngo Molnar <mingo@elte.hu>