nvme-pci: only call nvme_unmap_data for requests transferring data

This mirrors how nvme_map_pci is called and will allow simplifying some
checks in nvme_unmap_pci later on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

drivers/nvme/host/pci.c

@@ -929,7 +929,8 @@ static void nvme_pci_complete_rq(struct request *req)
 	struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
 
 	nvme_cleanup_cmd(req);
-	nvme_unmap_data(iod->nvmeq->dev, req);
+	if (blk_rq_nr_phys_segments(req))
+		nvme_unmap_data(iod->nvmeq->dev, req);
 	nvme_complete_rq(req);
 }