QoS: Use CLASSIFY iptables target instead of MARK

QoS: Use CLASSIFY iptables target instead of MARK

We have been running into loads of conflicts by using MARK for
various components on the OS (suricata, IPsec, QoS, ...) which
was sometimes hard to resolve.

iptables comes with a target which directly sorts packets into
the correct class which results in less code and not using the
mark.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>