nilfs2: move iterator to write log into segment buffer

This moves iterator to submit write requests for a series of logs into
segbuf.c, and hides nilfs_segbuf_write() and nilfs_segbuf_wait() in
the file.

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