nilfs2: disk address translator

This adds the disk address translation file (DAT) whose primary function
is to convert virtual disk block numbers to actual disk block numbers.

The virtual block numbers of NILFS are associated with checkpoint
generation numbers, and this file also provides functions to manage the
lifetime information of each virtual block number.

Signed-off-by: Koji Sato <sato.koji@lab.ntt.co.jp>
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>