net/packet: don't make IP6Header.marshalPseudo assume UDP

It will be used for ICMPv6 next, so pass in the proto.

Also, use the ipproto constants rather than hardcoding the mysterious
number.

Change-Id: I57b68bdd2d39fff75f82affe955aff9245de246b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/packet/ip6.go

@@ -57,7 +57,7 @@ func (h *IP6Header) ToResponse() {
 
 // marshalPseudo serializes h into buf in the "pseudo-header" form
 // required when calculating UDP checksums.
-func (h IP6Header) marshalPseudo(buf []byte) error {
+func (h IP6Header) marshalPseudo(buf []byte, proto ipproto.Proto) error {
 	if len(buf) < h.Len() {
 		return errSmallBuffer
 	}
@@ -72,6 +72,6 @@ func (h IP6Header) marshalPseudo(buf []byte) error {
 	buf[36] = 0
 	buf[37] = 0
 	buf[38] = 0
-	buf[39] = 17 // NextProto
+	buf[39] = byte(proto) // NextProto
 	return nil
 }

net/packet/udp6.go

@@ -40,7 +40,7 @@ func (h UDP6Header) Marshal(buf []byte) error {
 	binary.BigEndian.PutUint16(buf[46:48], 0) // blank checksum
 
 	// UDP checksum with IP pseudo header.
-	h.IP6Header.marshalPseudo(buf)
+	h.IP6Header.marshalPseudo(buf, ipproto.UDP)
 	binary.BigEndian.PutUint16(buf[46:48], ip4Checksum(buf[:]))
 
 	h.IP6Header.Marshal(buf)