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/mdt.h b/fs/nilfs2/mdt.h
index ab20a4b..ab172e8 100644
--- a/fs/nilfs2/mdt.h
+++ b/fs/nilfs2/mdt.h
@@ -28,6 +28,13 @@
 #include "nilfs.h"
 #include "page.h"
 
+/**
+ * struct nilfs_shadow_map - shadow mapping of meta data file
+ * @bmap_store: shadow copy of bmap state
+ * @frozen_data: shadowed dirty data pages
+ * @frozen_btnodes: shadowed dirty b-tree nodes' pages
+ * @frozen_buffers: list of frozen buffers
+ */
 struct nilfs_shadow_map {
 	struct nilfs_bmap_store bmap_store;
 	struct address_space frozen_data;