units: pull in loop.ko and dm-mod.ko before repart

units: pull in loop.ko and dm-mod.ko before repart

We want to make use of that when formatting file systems, hence let's
pull in these modules explicitly.

(This is necessary because we are an early boot service that might run
before systemd-tmpfiles-dev.service, which creates /dev/loop-control and
/dev/mapper/control.)

Alternatively we could just order ourselves after
systemd-tmpfiles-dev.service, but I think there's value in adding an
explicit minimal ordering here, since we know what we'll need.

Fixes: #25775