x86: add dma_capable() to replace is_buffer_dma_capable()

dma_capable() eventually replaces is_buffer_dma_capable(), which tells
if a memory area is dma-capable or not. The problem of
is_buffer_dma_capable() is that it doesn't take a pointer to struct
device so it doesn't work for POWERPC.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

arch/x86/include/asm/dma-mapping.h

@@ -55,6 +55,14 @@ extern int dma_set_mask(struct device *dev, u64 mask);
 extern void *dma_generic_alloc_coherent(struct device *dev, size_t size,
 					dma_addr_t *dma_addr, gfp_t flag);
 
+static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
+{
+	if (!dev->dma_mask)
+		return 0;
+
+	return addr + size <= *dev->dma_mask;
+}
+
 static inline void
 dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 	enum dma_data_direction dir)