net/tstun: implement env var for disabling UDP GRO on Linux (#11924)

Certain device drivers (e.g. vxlan, geneve) do not properly handle
coalesced UDP packets later in the stack, resulting in packet loss.

Updates #11026

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

net/tstun/tun_features_linux.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tstun
+
+import (
+	"github.com/tailscale/wireguard-go/tun"
+	"tailscale.com/envknob"
+)
+
+func setLinkFeatures(dev tun.Device) error {
+	if envknob.Bool("TS_TUN_DISABLE_UDP_GRO") {
+		linuxDev, ok := dev.(tun.LinuxDevice)
+		if ok {
+			linuxDev.DisableUDPGRO()
+		}
+	}
+	return nil
+}