nilfs2: add iterator for segment buffers

This adds a few iterator functions for segment buffers to make it easy
to handle multiple series of logs.

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