arm64: dma-mapping: Fix FORCE_CONTIGUOUS buffer clearing

We need to invalidate the caches *before* clearing the buffer via the
non-cacheable alias, else in the worst case __dma_flush_area() may
write back dirty lines over the top of our nice new zeros.

Fixes: dd65a941 ("arm64: dma-mapping: clear buffers allocated with FORCE_CONTIGUOUS flag")
Cc: <stable@vger.kernel.org> # 4.18.x-
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/dma-mapping.c

@@ -429,9 +429,9 @@ static void *__iommu_alloc_attrs(struct device *dev, size_t size,
 						   prot,
 						   __builtin_return_address(0));
 		if (addr) {
-			memset(addr, 0, size);
 			if (!coherent)
 				__dma_flush_area(page_to_virt(page), iosize);
+			memset(addr, 0, size);
 		} else {
 			iommu_dma_unmap_page(dev, *handle, iosize, 0, attrs);
 			dma_release_from_contiguous(dev, page,