Support multicast at the KNI interfaces

Some ASes would prefer to run OSPF and BGP on BIRD on Gatekeeper servers, and OSPF relies on multicast.

Here's the log of two of OSPF's packets:
```
[2022-11-26 14:38:27] GK/5: acl: a packet failed to match any ACL rules, the whole packet is dumped below:
dump mbuf at 0x21ffd49b00, iova=0x21ffd49b98, buf_len=2176
pkt_len=62, ol_flags=0x182, nb_segs=1, port=0, ptype=0x290
segment at 0x21ffd49b00, data=0x21ffd49c18, len=62, off=128, refcnt=1
Dump data at [0x21ffd49c18], len=62
00000000: 6C B3 11 21 7E 84 60 12 3C 41 F9 42 08 00 45 00 | l..!~.`.