blk-mq: only dispatch to non-defauly queue maps if they have queues

We should check if a given queue map actually has queues enabled before
dispatching to it.  This allows drivers to not initialize optional but
not used map types, which subsequently will allow fixing problems with
queue map rebuilds for that case.
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.h

@@ -105,12 +105,15 @@ static inline struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *q,
 {
 	enum hctx_type type = HCTX_TYPE_DEFAULT;
 
-	if (q->tag_set->nr_maps > HCTX_TYPE_POLL &&
-	    ((flags & REQ_HIPRI) && test_bit(QUEUE_FLAG_POLL, &q->queue_flags)))
+	if ((flags & REQ_HIPRI) &&
+	    q->tag_set->nr_maps > HCTX_TYPE_POLL && 
+	    q->tag_set->map[HCTX_TYPE_POLL].nr_queues &&
+	    test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
 		type = HCTX_TYPE_POLL;
 
-	else if (q->tag_set->nr_maps > HCTX_TYPE_READ &&
-		 ((flags & REQ_OP_MASK) == REQ_OP_READ))
+	else if (((flags & REQ_OP_MASK) == REQ_OP_READ) &&
+	         q->tag_set->nr_maps > HCTX_TYPE_READ &&
+		 q->tag_set->map[HCTX_TYPE_READ].nr_queues)
 		type = HCTX_TYPE_READ;
 	
 	return blk_mq_map_queue_type(q, type, cpu);