nilfs2: move routine to set segment usage into sufile

This adds nilfs_sufile_set_segment_usage() function in sufile to
replace direct access to the sufile metadata in log writer code.

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

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

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

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