swiotlb: remove the overflow buffer

Like all other dma mapping drivers just return an error code instead
of an actual memory buffer.  The reason for the overflow buffer was
that at the time swiotlb was invented there was no way to check for
dma mapping errors, but this has long been fixed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

include/linux/dma-direct.h

@@ -5,6 +5,8 @@
 #include <linux/dma-mapping.h>
 #include <linux/mem_encrypt.h>
 
+#define DIRECT_MAPPING_ERROR		0
+
 #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
 #include <asm/dma-direct.h>
 #else

include/linux/swiotlb.h

@@ -106,9 +106,6 @@ extern void
 swiotlb_sync_sg_for_device(struct device *hwdev, struct scatterlist *sg,
 			   int nelems, enum dma_data_direction dir);
 
-extern int
-swiotlb_dma_mapping_error(struct device *hwdev, dma_addr_t dma_addr);
-
 extern int
 swiotlb_dma_supported(struct device *hwdev, u64 mask);