nilfs-utils: v2.1.6 release

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: fix build of dist archives

The following files are missing in the archive files generated by
"make dist-*":

 - .gitignore files
 - autogen.sh script
 - checkpatch.pl script

This fixes the issue.

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