blk-mq: don't use preempt_count() to check for right CPU

UP or CONFIG_PREEMPT_NONE will return 0, and what we really
want to check is whether or not we are on the right CPU.
So don't make PREEMPT part of this, just test the CPU in
the mask directly.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -534,7 +534,7 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 	LIST_HEAD(rq_list);
 	int bit, queued;
 
-	WARN_ON(!preempt_count());
+	WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask));
 
 	if (unlikely(test_bit(BLK_MQ_S_STOPPED, &hctx->state)))
 		return;