udev: when booting without root= specification, and searching a root partition...

udev: when booting without root= specification, and searching a root partition actually do the version comparison magic

Since 08fe0a53 when dissecting a disk
image we'll automatically pick the "newest" root fs if multiple exist,
by comparing GPT partition labels. This works in systemd-nspawn,
systemd-dissect, systemd-tmpfiles --image, … and so on. It also works
already in systemd-gpt-auto-generator. However, there was one missing
place: in the logic that automatically finds a root fs in case no root=
was specified on the kernel logic at all. This logic doesn't use the
dissection logic, but a much simpler one.

Let's fill the gap, and implement it there too.