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>