When we hot-unplug a cpu and rebuild the sched-domain, all cpus will be
detatched. Alex observed the case where a runqueue was stealing bandwidth
from an already disabled runqueue to satisfy its own needs.

Stop this by skipping over already disabled runqueues.
Reported-by: NAlex Nixon <alex.nixon@citrix.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: NAlex Nixon <alex.nixon@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>