nilfs2: move out checksum routines to segment buffer code

This moves out checksum routines in log writer to segbuf.c for
cleanup.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

diff --cc fs/nilfs2/segbuf.c
Simple merge

diff --cc fs/nilfs2/segbuf.h
Simple merge

diff --cc fs/nilfs2/segment.c
Simple merge