cfq-iosched: fix corner cases in idling logic

Idling logic was disabled in some corner cases, leading to unfair share
 for noidle queues.
 * the idle timer was not armed if there were other requests in the
   driver. unfortunately, those requests could come from other workloads,
   or queues for which we don't enable idling. So we will check only
   pending requests from the active queue
 * rq_noidle check on no-idle queue could disable the end of tree idle if
   the last completed request was rq_noidle. Now, we will disable that
   idle only if all the queues served in the no-idle tree had rq_noidle
   requests.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>