nilfs2: implement nilfs_show_options to display mount options in /proc/mounts

This is a patch to display mount options in procfs.
Mount options will show up in the /proc/mounts as other fs does.

...
/dev/sda6 /mnt nilfs2 ro,relatime,barrier=off,cp=3,order=strict 0 0
...

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