udev: when setting up lo, do not return an error

From #20300:
> commit 70f32a26
> Author: Yu Watanabe <watanabe.yu+github@gmail.com>
> Date:   Sun May 23 16:59:40 2021 +0900

>     udev/net: do not manage loopback interfaces

>     There are nothing we can configure in udevd for loopback interfaces;
>     no ethertool configs can be applied, MAC address, interface name should

> introduced a regression for 'udevadm test-builtin net_setup_link /sys/class/net/lo/'.
> Prior to this commit this command would exit with 0 whereas after this commit
> it exists with 1. This causes cloud-init on Archlinux to fail as this command
> is run by it and likely also netplan to have networkd rescan and re-apply a
> bunch of things on NICs.

I think it's reasonable to keep returning 0 here: we are intentatinally doing
nothing for the device, and that is not an error, but a (noop) success.

Fixes #20300.

(cherry picked from commit b4f02613)