vendor: Update golang.org/x/net/ipv6

vendor/golang.org/x/net/ipv6/payload.go

@@ -1,4 +1,4 @@
-// Copyright 2013 The Go Authors.  All rights reserved.
+// Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
@@ -6,6 +6,9 @@ package ipv6
 
 import "net"
 
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
 // A payloadHandler represents the IPv6 datagram payload handler.
 type payloadHandler struct {
 	net.PacketConn