Yup, it seems that all along I've just missed that we could have
IPv6Packet:s in `ip_packet`, and their source is accessed by
`.ipv6_saddr`, not `ip_saddr` (that's for IPv4Packet). So, let's just
try and see which one of the two each `ip_packet` has, because one of
them must be there!

Also, given that UDPPacket can be either IPv4 or IPv6 it seems safest
to try to parse each packet as IPv6Packet first -- that way we keep
looking at transport layer protocols for IPv4 only, and treat
everything IPv6 as the same, which makes sense, since we should block
all IPv6, so everything should be treated the same at all times.

Refs: #11508