iommu: add dma_get_mask helper function

Several IOMMUs do the same thing to get the dma_mask of a device. This
adds a helper function to do the same thing to sweep them.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/dma-mapping.h

@@ -63,6 +63,13 @@ static inline int is_buffer_dma_capable(u64 mask, dma_addr_t addr, size_t size)
 #define dma_sync_single		dma_sync_single_for_cpu
 #define dma_sync_sg		dma_sync_sg_for_cpu
 
+static inline u64 dma_get_mask(struct device *dev)
+{
+	if (dev->dma_mask && *dev->dma_mask)
+		return *dev->dma_mask;
+	return DMA_32BIT_MASK;
+}
+
 extern u64 dma_get_required_mask(struct device *dev);
 
 static inline unsigned int dma_get_max_seg_size(struct device *dev)