f2fs: introduce help function F2FS_NODE()

Introduce help function F2FS_NODE() to simplify the conversion of node_page to
f2fs_node.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index 2b2d45d1..debf743 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -56,7 +56,7 @@
 	if (IS_ERR(node_page))
 		return PTR_ERR(node_page);
 
-	rn = page_address(node_page);
+	rn = F2FS_NODE(node_page);
 	ri = &(rn->i);
 
 	inode->i_mode = le16_to_cpu(ri->i_mode);
@@ -153,7 +153,7 @@
 
 	wait_on_page_writeback(node_page);
 
-	rn = page_address(node_page);
+	rn = F2FS_NODE(node_page);
 	ri = &(rn->i);
 
 	ri->i_mode = cpu_to_le16(inode->i_mode);