mcproxy: fix block/filtering code

mcproxy: fix block/filtering code

mcproxy supports a generic filtering/blacklisting mechanism but it’s currently
broken. In the case of routed video (e.g. mcproxy routing video from
WAN -> LAN), it will forward multicast between the LAN and WAN. There are
perfectly valid use cases for this like reporting but other less-appropriate
things like mDNS and SSDP leak through from LAN -> WAN which is bad.

Signed-off-by: Sukru Senli <sukru.senli@iopsys.eu>
Signed-off-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: John Crispin <john@phrozen.org>