tunefs.lustre exits with rc 0 without writing anything when --mountfsoptions
is the only change, silently discarding the new mount options (old options
still used on next mount). 

Reproduce:

tunefs.lustre --mountfsoptions=<new opts> /dev/XXX  (no other option)
then --print shows the opts unchanged. Adding any other option (e.g. --param)
makes it persist.

Found by commit-hunter-v1.