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.c b/fs/f2fs/node.c
index 0af0a71..e8fe52d 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -393,8 +393,8 @@
 
 /*
  * Caller should call f2fs_put_dnode(dn).
- * Also, it should grab and release a mutex by calling mutex_lock_op() and
- * mutex_unlock_op() only if ro is not set RDONLY_NODE.
+ * Also, it should grab and release a rwsem by calling f2fs_lock_op() and
+ * f2fs_unlock_op() only if ro is not set RDONLY_NODE.
  * In the case of RDONLY_NODE, we don't need to care about mutex.
  */
 int get_dnode_of_data(struct dnode_of_data *dn, pgoff_t index, int mode)
@@ -802,8 +802,8 @@
 }
 
 /*
- * Caller should grab and release a mutex by calling mutex_lock_op() and
- * mutex_unlock_op().
+ * Caller should grab and release a rwsem by calling f2fs_lock_op() and
+ * f2fs_unlock_op().
  */
 void remove_inode_page(struct inode *inode)
 {