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>

fs/f2fs/file.c

@@ -206,7 +206,7 @@ int truncate_data_blocks_range(struct dnode_of_data *dn, int count)
 	struct f2fs_node *raw_node;
 	__le32 *addr;
 
-	raw_node = page_address(dn->node_page);
+	raw_node = F2FS_NODE(dn->node_page);
 	addr = blkaddr_in_node(raw_node) + ofs;
 
 	for ( ; count > 0; count--, addr++, dn->ofs_in_node++) {