f2fs: introduce NODE_MAPPING for code consistency

This patch adds NODE_MAPPING which is similar as META_MAPPING introduced by
Gu Zheng.

Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 117e30f..af51a0b 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -538,6 +538,11 @@
 	return sbi->meta_inode->i_mapping;
 }
 
+static inline struct address_space *NODE_MAPPING(struct f2fs_sb_info *sbi)
+{
+	return sbi->node_inode->i_mapping;
+}
+
 static inline void F2FS_SET_SB_DIRT(struct f2fs_sb_info *sbi)
 {
 	sbi->s_dirty = 1;