pearing off zone from physical memory layout [2/10] add helper function page_node()

Index: hogehoge/include/linux/mm.h
===================================================================
@@ -478,6 +478,12 @@ static inline unsigned long page_to_nid(
 	else
 		return page_zone(page)->zone_pgdat->node_id;
 }
+
+static inline struct pglist_data *page_node(struct page *page)
+{
+	return NODE_DATA(page_to_nid(page));
+}
+
 static inline unsigned long page_to_section(struct page *page)
 {
 	return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;