nilfs2: checkpoint file

This adds a meta data file which holds checkpoint entries in its data
blocks.

Signed-off-by: Koji Sato <[email protected]>
Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Koji Sato