x86: don't do dma if mask is NULL.

if the device hasn't provided a mask, abort allocation.
Note that we're using a fallback device now, so it does not cover
the case of a NULL device: just drivers passing NULL masks around.
Signed-off-by: NGlauber Costa <gcosta@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/pci-dma_32.c

@@ -91,6 +91,9 @@ void *dma_alloc_coherent(struct device *dev, size_t size,
 	if (dma_mask == 0)
 		dma_mask = DMA_32BIT_MASK;
 
+	if (dev->dma_mask == NULL)
+		return NULL;
+
 	/* Don't invoke OOM killer */
 	gfp |= __GFP_NORETRY;
 again: