x86/dma-mapping: Fix arch_dma_alloc_attrs() oops with NULL dev

Commit 6894258eda2f broke drivers that pass NULL as the device pointer
to dma_alloc. The reason is that arch_dma_alloc_attrs() now calls
dma_alloc_coherent_gfp_flags() which in turn calls
dma_alloc_coherent_mask(), where the device pointer is dereferenced
unconditionally.

Fix things by moving the ISA DMA fallback device assignment before the
call to dma_alloc_coherent_gfp_flags().

Fixes: 6894258eda2f ("dma-mapping: consolidate dma_{alloc,free}_{attrs,coherent}")
Reported-and-tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/1445807503-8920-1-git-send-email-ville.syrjala@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/pci-dma.c

@@ -131,11 +131,12 @@ void dma_generic_free_coherent(struct device *dev, size_t size, void *vaddr,
 
 bool arch_dma_alloc_attrs(struct device **dev, gfp_t *gfp)
 {
+	if (!*dev)
+		*dev = &x86_dma_fallback_dev;
+
 	*gfp &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);
 	*gfp = dma_alloc_coherent_gfp_flags(*dev, *gfp);
 
-	if (!*dev)
-		*dev = &x86_dma_fallback_dev;
 	if (!is_device_dma_capable(*dev))
 		return false;
 	return true;