f2fs: update several comments

Update several comments:
1. use f2fs_{un}lock_op install of mutex_{un}lock_op.
2. update comment of get_data_block().
3. update description of node offset.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/node.h b/fs/f2fs/node.h
index 3496bb3..c4c7988 100644
--- a/fs/f2fs/node.h
+++ b/fs/f2fs/node.h
@@ -224,7 +224,13 @@
  *    |            `- direct node (5 + N => 5 + 2N - 1)
  *    `- double indirect node (5 + 2N)
  *                 `- indirect node (6 + 2N)
- *                       `- direct node (x(N + 1))
+ *                       `- direct node
+ *                 ......
+ *                 `- indirect node ((6 + 2N) + x(N + 1))
+ *                       `- direct node
+ *                 ......
+ *                 `- indirect node ((6 + 2N) + (N - 1)(N + 1))
+ *                       `- direct node
  */
 static inline bool IS_DNODE(struct page *node_page)
 {