nilfs2: change return type of nilfs_commit_chunk

change return type of nilfs_commit_chunk() as void from int,
for nilfs_set_file_dirty() usually does not return error.

This is an intermediate patch to reduce mark_inode_dirty() in
nilfs_commit_chunk().

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