swiotlb: replace dma_length with sg_dma_len() macro

This patch replace dma_length in "lib/swiotlb.c" to sg_dma_len() macro,
because the build error can occur if CONFIG_NEED_SG_DMA_LENGTH is not
set, and CONFIG_SWIOTLB is set.

Singed-off-by: EunBong Song <[e-mail]>
Signed-off-by: Stefano Stabellini <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>

lib/swiotlb.c

···
 				swiotlb_full(hwdev, sg->length, dir, 0);
 				swiotlb_unmap_sg_attrs(hwdev, sgl, i, dir,
 						       attrs);
-				sgl[0].dma_length = 0;
+				sg_dma_len(sgl) = 0;
 				return 0;
 			}
 			sg->dma_address = phys_to_dma(hwdev, map);
 		} else
 			sg->dma_address = dev_addr;
-		sg->dma_length = sg->length;
+		sg_dma_len(sg) = sg->length;
 	}
 	return nelems;
 }
···
 	BUG_ON(dir == DMA_NONE);
 
 	for_each_sg(sgl, sg, nelems, i)
-		unmap_single(hwdev, sg->dma_address, sg->dma_length, dir);
+		unmap_single(hwdev, sg->dma_address, sg_dma_len(sg), dir);
 
 }
 EXPORT_SYMBOL(swiotlb_unmap_sg_attrs);
···
 
 	for_each_sg(sgl, sg, nelems, i)
 		swiotlb_sync_single(hwdev, sg->dma_address,
-				    sg->dma_length, dir, target);
+				    sg_dma_len(sg), dir, target);
 }
 
 void