net/{packet,tstun},wgengine: update disco key when receiving via TSMP (#18158)

When receiving a TSMPDiscoAdvertisement from peer, update the discokey
for said peer.

Some parts taken from: https://github.com/tailscale/tailscale/pull/18073/

Updates #12639

Co-authored-by: James Tucker <james@tailscale.com>

net/packet/tsmp.go

@@ -271,7 +271,7 @@ func (h TSMPPongReply) Marshal(buf []byte) error {
 //   - 'a' (TSMPTypeDiscoAdvertisement)
 //   - 32 disco key bytes
 type TSMPDiscoKeyAdvertisement struct {
-	Src, Dst netip.Addr
+	Src, Dst netip.Addr // Src and Dst are set from the parent IP Header when parsing.
 	Key      key.DiscoPublic
 }
 
@@ -298,7 +298,7 @@ func (ka *TSMPDiscoKeyAdvertisement) Marshal() ([]byte, error) {
 		return []byte{}, fmt.Errorf("expected payload length 33, got %d", len(payload))
 	}
 
-	return Generate(iph, payload), nil
+	return Generate(iph, payload[:]), nil
 }
 
 func (pp *Parsed) AsTSMPDiscoAdvertisement() (tka TSMPDiscoKeyAdvertisement, ok bool) {
@@ -310,6 +310,7 @@ func (pp *Parsed) AsTSMPDiscoAdvertisement() (tka TSMPDiscoKeyAdvertisement, ok
 		return
 	}
 	tka.Src = pp.Src.Addr()
+	tka.Dst = pp.Dst.Addr()
 	tka.Key = key.DiscoPublicFromRaw32(mem.B(p[1:33]))
 
 	return tka, true