dma-direct: fail allocations that can't be made coherent

If the architecture can't remap or set an address uncached there is no way
to fullfill a request for a coherent allocation.  Return NULL in that case.
Note that this case currently does not happen, so this is a theoretical
fixup and/or a preparation for eventually supporting platforms that
can't support coherent allocations with the generic code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

kernel/dma/direct.c

@@ -233,8 +233,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 				return dma_direct_alloc_from_pool(dev, size,
 						dma_handle, gfp);
 		} else {
-			if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED))
-				set_uncached = true;
+			if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED))
+				return NULL;
+			set_uncached = true;
 		}
 	}