swiotlb-xen: replace dma_length with sg_dma_len() macro

swiotlb-xen has an implicit dependency on CONFIG_NEED_SG_DMA_LENGTH.
Remove it by replacing dma_length with sg_dma_len.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/swiotlb-xen.c

@@ -506,13 +506,13 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
 				   to do proper error handling. */
 				xen_swiotlb_unmap_sg_attrs(hwdev, sgl, i, dir,
 							   attrs);
-				sgl[0].dma_length = 0;
+				sg_dma_len(sgl) = 0;
 				return DMA_ERROR_CODE;
 			}
 			sg->dma_address = xen_phys_to_bus(map);
 		} else
 			sg->dma_address = dev_addr;
-		sg->dma_length = sg->length;
+		sg_dma_len(sg) = sg->length;
 	}
 	return nelems;
 }
@@ -533,7 +533,7 @@ xen_swiotlb_unmap_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
 	BUG_ON(dir == DMA_NONE);
 
 	for_each_sg(sgl, sg, nelems, i)
-		xen_unmap_single(hwdev, sg->dma_address, sg->dma_length, dir);
+		xen_unmap_single(hwdev, sg->dma_address, sg_dma_len(sg), dir);
 
 }
 EXPORT_SYMBOL_GPL(xen_swiotlb_unmap_sg_attrs);
@@ -555,7 +555,7 @@ xen_swiotlb_sync_sg(struct device *hwdev, struct scatterlist *sgl,
 
 	for_each_sg(sgl, sg, nelems, i)
 		xen_swiotlb_sync_single(hwdev, sg->dma_address,
-					sg->dma_length, dir, target);
+					sg_dma_len(sg), dir, target);
 }
 
 void