nilfs2: add struct nilfs_suinfo_update and flags

Add the nilfs_suinfo_update structure, which contains the information
needed to update one segment usage entry.  The flags specify, which
fields need to be updated.

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>
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>