nilfs2: separate function that updates log position

This moves out section that updates information of the recent log
position stored in super blocks from nilfs_commit_super to a new
routine named nilfs_set_log_cursor.

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

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

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