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

This reverts commit 0298d543.

With this patch, set 'poll_queues > hard queues' will lead to 'nr_read_queues = 0'
in nvme_calc_irq_sets. Then poll_queues setting can fail since dev->tagset.nr_maps
equals to 2 and nvme_pci_map_queues will not do map for poll queues.
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/pci.c

@@ -2254,9 +2254,7 @@ 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 = 1; /* default */
-		if (dev->io_queues[HCTX_TYPE_READ])
-			dev->tagset.nr_maps++;
+		dev->tagset.nr_maps = 2; /* default + read */
 		if (dev->io_queues[HCTX_TYPE_POLL])
 			dev->tagset.nr_maps++;
 		dev->tagset.timeout = NVME_IO_TIMEOUT;