fstab-generator: use RequiresOverridable for fsck units

fstab-generator: use RequiresOverridable for fsck units

This allows the user to disable fsck's by masking.

If fsck fails, emergency target is started, the user might mount the
unit using mount and disable fsck by masking the unit. In this case,
.mount will be active because the mount is detect through
/proc/self/mountinfo, but systemd-fsck@.service will still be in
failed mode. This results in a funny situation where

$ systemctl show -p ActiveState local-fs.target yyy.mount
ActiveState=active
ActiveState=active

$ sudo systemctl start local-fs.target
[sudo] password for test:
Failed to start local-fs.target: Unit systemd-fsck@xxx.service is masked.