nilfs2: hide nilfs_write_info struct in segment buffer code

Hides nilfs_write_info struct and nilfs_segbuf_prepare_write function
in segbuf.c to simplify the interface of nilfs_segbuf_write function.

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