powerpc: Fix bug in iommu_alloc_coherent causing hang during boot

In commit 8eb6c6e3b9c8bfed3d75536ab142d7694627c2e5, 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>
diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index cbb7945..cef8cba 100644
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -561,7 +561,7 @@
 		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);