blk-mq: improve warning for running a queue on the wrong CPU

__blk_mq_run_hw_queue() currently warns if we are running the queue on a
CPU that isn't set in its mask. However, this can happen if a CPU is
being offlined, and the workqueue handling will place the work on CPU0
instead. Improve the warning so that it only triggers if the batch cpu
in the hardware queue is currently online.  If it triggers for that
case, then it's indicative of a flow problem in blk-mq, so we want to
retain it for that case.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -793,11 +793,12 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 	struct list_head *dptr;
 	int queued;
 
-	WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask));
-
 	if (unlikely(test_bit(BLK_MQ_S_STOPPED, &hctx->state)))
 		return;
 
+	WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
+		cpu_online(hctx->next_cpu));
+
 	hctx->run++;
 
 	/*