Comment # 5 on bug 968573 from Christian Boltz

Hmm, why do you manually create a symlink at all?

AFAIK it should work if you just do
    systemctl enable mgetty@modem.service
_without_ creating any symlink or hardlink before.

The "systemctl enable" command should create a symlink like  *)
/etc/systemd/system/multi-user.target.wants/mgetty@modem.service ->
/usr/lib/systemd/system/mgetty@.service

*) I didn't check which target is used for mgetty@, so the symlink might 
   end up in another /etc/systemd/system/*.target.wants directory