nvme-pci: don't create a read hctx mapping without read queues

Only request an IRQ mapping for read queues if at least one read queue
is being allocted, as nvme_pci_map_queues() will later on ignore the
unnecessary mapping request should nvme_dev_add() request such an IRQ
mapping even though no read queues are being allocated.  However,
nvme_dev_add() can avoid making the request by checking the number of
read queues without assuming. This would bring it more in line with
nvme_setup_irqs() and nvme_calc_irq_sets().
Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/pci.c

@@ -2250,7 +2250,9 @@ static int nvme_dev_add(struct nvme_dev *dev)
 	if (!dev->ctrl.tagset) {
 		dev->tagset.ops = &nvme_mq_ops;
 		dev->tagset.nr_hw_queues = dev->online_queues - 1;
-		dev->tagset.nr_maps = 2; /* default + read */
+		dev->tagset.nr_maps = 1; /* default */
+		if (dev->io_queues[HCTX_TYPE_READ])
+			dev->tagset.nr_maps++;
 		if (dev->io_queues[HCTX_TYPE_POLL])
 			dev->tagset.nr_maps++;
 		dev->tagset.timeout = NVME_IO_TIMEOUT;