wgengine/magicsock: refactor batchingUDPConn to batchingConn interface (#13042)

This commit adds a batchingConn interface, and renames batchingUDPConn
to linuxBatchingConn. tryUpgradeToBatchingConn() may return a platform-
specific implementation of batchingConn. So far only a Linux
implementation of this interface exists, but this refactor is being
done in anticipation of a Windows implementation.

Updates tailscale/corp#21874

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

wgengine/magicsock/batching_conn_default.go

@@ -0,0 +1,14 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package magicsock
+
+import (
+	"tailscale.com/types/nettype"
+)
+
+func tryUpgradeToBatchingConn(pconn nettype.PacketConn, _ string, _ int) nettype.PacketConn {
+	return pconn
+}