net/packet: implement Geneve header serialization (#15301)

Updates tailscale/corp#27100

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/packet/geneve_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package packet
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestGeneveHeader(t *testing.T) {
+	in := GeneveHeader{
+		Version:  3,
+		Protocol: GeneveProtocolDisco,
+		VNI:      1<<24 - 1,
+		Control:  true,
+	}
+	b := make([]byte, GeneveFixedHeaderLength)
+	err := in.Encode(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := GeneveHeader{}
+	err = out.Decode(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(out, in); diff != "" {
+		t.Fatalf("wrong results (-got +want)\n%s", diff)
+	}
+}