dma-direct: comment the dma_direct_free calling convention

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

lib/dma-direct.c

@@ -84,6 +84,10 @@ again:
 	return page_address(page);
 }
 
+/*
+ * NOTE: this function must never look at the dma_addr argument, because we want
+ * to be able to use it as a helper for iommu implementations as well.
+ */
 void dma_direct_free(struct device *dev, size_t size, void *cpu_addr,
 		dma_addr_t dma_addr, unsigned long attrs)
 {