nilfs2: add omitted comments for different structures in driver implementation

Add omitted comments for different structures in driver implementation.

Signed-off-by: Vyacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/nilfs2/btnode.h b/fs/nilfs2/btnode.h
index 3a4dd2d..d876b56 100644
--- a/fs/nilfs2/btnode.h
+++ b/fs/nilfs2/btnode.h
@@ -29,7 +29,13 @@
 #include <linux/fs.h>
 #include <linux/backing-dev.h>
 
-
+/**
+ * struct nilfs_btnode_chkey_ctxt - change key context
+ * @oldkey: old key of block's moving content
+ * @newkey: new key for block's content
+ * @bh: buffer head of old buffer
+ * @newbh: buffer head of new buffer
+ */
 struct nilfs_btnode_chkey_ctxt {
 	__u64 oldkey;
 	__u64 newkey;