nilfs2: segment buffer

This adds the segment buffer which is used to constuct logs.

[[email protected]: BIO_RW_SYNC got removed]
Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: konis