powerpc: Fix bug in iommu_alloc_coherent causing hang during boot

In commit 8eb6c6e3, Christoph Hellwig
made iommu_alloc_coherent able to do node-local allocations, but
unfortunately got the order of the arguments to alloc_pages_node
wrong.  This fixes it.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/iommu.c

@@ -561,7 +561,7 @@ void *iommu_alloc_coherent(struct iommu_table *tbl, size_t size,
 		return NULL;
 
 	/* Alloc enough pages (and possibly more) */
-	page = alloc_pages_node(flag, order, node);
+	page = alloc_pages_node(node, flag, order);
 	if (!page)
 		return NULL;
 	ret = page_address(page);