kernel-netlink: Avoid route dump if routing rule excludes traffic with a certain...

kernel-netlink: Avoid route dump if routing rule excludes traffic with a certain mark

If the routing rule we use to direct traffic to our own routing table
excludes traffic with a certain mark (fwmark = !<mark>) we can simplify
the route lookup and avoid dumping all routes by passing the mark to the
request.  That way our own routes are ignored and we get the preferred
route back without having to dump and analyze all routes, which is quite a
burden on hosts with lots of routes.