nilfs2: relocate io status variables to segment buffer

This moves io status variables in nilfs_write_info struct to
nilfs_segment_buffer struct.

This is a preparation to hide nilfs_write_info in segment buffer code.

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