net/{batching,packet},wgengine/magicsock: export batchingConn (#16848)

For eventual use by net/udprelay.Server.

Updates tailscale/corp#31164

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

net/batching/conn_default.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package batching
+
+import (
+	"tailscale.com/types/nettype"
+)
+
+// TryUpgradeToConn is no-op on all platforms except linux.
+func TryUpgradeToConn(pconn nettype.PacketConn, _ string, _ int) nettype.PacketConn {
+	return pconn
+}
+
+var controlMessageSize = 0
+
+func MinControlMessageSize() int {
+	return controlMessageSize
+}