nilfs2: cleanup multi kmem_cache_{create,destroy} code

This cleanup patch gives several improvements:

 - Moving all kmem_cache_{create_destroy} calls into one place, which removes
 some small function calls, cleans up error check code and clarify the logic.

 - Mark all initial code in __init section.

 - Remove some very obvious comments.

 - Adjust some declarations.

 - Fix some space-tab issues.

Signed-off-by: Li Hong <lihong.hi@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/segment.h b/fs/nilfs2/segment.h
index e61fc79..7aca765 100644
--- a/fs/nilfs2/segment.h
+++ b/fs/nilfs2/segment.h
@@ -217,6 +217,8 @@
  */
 #define NILFS_SC_DEFAULT_WATERMARK  3600
 
+/* super.c */
+extern struct kmem_cache *nilfs_transaction_cachep;
 
 /* segment.c */
 extern int nilfs_init_transaction_cache(void);