net/packet: support full IPv6 decoding.

The packet filter still rejects all IPv6, but decodes enough from v6
packets to do something smarter in a followup.

name              time/op
Decode/tcp4-8     28.8ns ± 2%
Decode/tcp6-8     20.6ns ± 1%
Decode/udp4-8     28.2ns ± 1%
Decode/udp6-8     20.0ns ± 6%
Decode/icmp4-8    21.7ns ± 2%
Decode/icmp6-8    14.1ns ± 2%
Decode/unknown-8  9.43ns ± 2%

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/filter/filter.go

@@ -243,7 +243,7 @@ func (f *Filter) runIn(q *packet.Parsed) (r Response, why string) {
 	}
 
 	switch q.IPProto {
-	case packet.ICMP:
+	case packet.ICMPv4:
 		if q.IsEchoResponse() || q.IsError() {
 			// ICMP responses are allowed.
 			// TODO(apenwarr): consider using conntrack state.
@@ -383,7 +383,7 @@ func omitDropLogging(p *packet.Parsed, dir direction) bool {
 			// it doesn't know about, so parse it out ourselves if needed.
 			ipProto := p.IPProto
 			if ipProto == 0 && len(b) > 8 {
-				ipProto = packet.IP4Proto(b[9])
+				ipProto = packet.IPProto(b[9])
 			}
 			// Omit logging about outgoing IGMP.
 			if ipProto == packet.IGMP {