x86/dma: Remove the x86_dma_fallback_dev hack

Now that we removed support for the NULL device argument in the DMA API,
there is no need to cater for that in the x86 code.

Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/mapping.c

@@ -238,10 +238,6 @@ u64 dma_get_required_mask(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(dma_get_required_mask);
 
-#ifndef arch_dma_alloc_attrs
-#define arch_dma_alloc_attrs(dev)	(true)
-#endif
-
 void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
 		gfp_t flag, unsigned long attrs)
 {
@@ -256,9 +252,6 @@ void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
 	/* let the implementation decide on the zone to allocate from: */
 	flag &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
 
-	if (!arch_dma_alloc_attrs(&dev))
-		return NULL;
-
 	if (dma_is_direct(ops))
 		cpu_addr = dma_direct_alloc(dev, size, dma_handle, flag, attrs);
 	else if (ops->alloc)