nvme-pci: fix freeing single sgl

There may only be a single DMA mapped entry from multiple physical
segments, which means we don't allocate a separte SGL list. Check the
number of allocations prior to know if we need to free something.

Freeing a single list allocation is the same for both PRP and SGL
usages, so we don't need to check the use_sgl flag anymore.

Fixes: 01df742d ("nvme-pci: remove SGL segment descriptors")
Reported-by: NNiklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: NKeith Busch <kbusch@kernel.org>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NNiklas Schnelle <schnelle@linux.ibm.com>

drivers/nvme/host/pci.c

@@ -556,7 +556,7 @@ static void nvme_unmap_data(struct nvme_dev *dev, struct request *req)
 	if (iod->nr_allocations == 0)
 		dma_pool_free(dev->prp_small_pool, iod->list[0].sg_list,
 			      iod->first_dma);
-	else if (iod->use_sgl)
+	else if (iod->nr_allocations == 1)
 		dma_pool_free(dev->prp_page_pool, iod->list[0].sg_list,
 			      iod->first_dma);
 	else