tmpfiles: don't adjust qgroups on existing subvolumes

tmpfiles: don't adjust qgroups on existing subvolumes

The qgroup logic (types 'q' and 'Q') only has an effect if there's no previous
setup at all, and any explicitly configured subvolumes with their qgroups are
left entirely unmodified.

The idea is that if users want a different logic than the one we set up by
default, then by all means they should do that before hand, and tmpfiles won't
override their logic.