nilfs2: add update functions of virtual block address to dat

This is a preparation for the successive cleanup ("nilfs2: allow btree
to directly call dat operations").

This adds functions bundling a few operations to change an entry of
virtual block address on the dat file.

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

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

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

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