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.go

@@ -53,6 +53,9 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 		dev.Close()
 		return nil, "", err
 	}
+	if err := setLinkFeatures(dev); err != nil {
+		logf("setting link features: %v", err)
+	}
 	if err := setLinkAttrs(dev); err != nil {
 		logf("setting link attributes: %v", err)
 	}