blkio: Set must_dispatch only if we decided to not dispatch the request

o must_dispatch flag should be set only if we decided not to run the queue
  and dispatch the request.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -2491,7 +2491,7 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 			    cfqd->busy_queues > 1) {
 				del_timer(&cfqd->idle_slice_timer);
 				__blk_run_queue(cfqd->queue);
-			}
+			} else
 				cfq_mark_cfqq_must_dispatch(cfqq);
 		}
 	} else if (cfq_should_preempt(cfqd, cfqq, rq)) {