Issue title: DS918+ interface speed only 10Mb/s
 Issue body: - Model: DS918+
- Current DSM version: 6.2.3
- Installed Zerotier version :zerotier_x64-6.1_1.4.0-0.spk

all function work well , but zerotier interface speed only 10Mb/s. physical interface has 1000Mb/s.

![image](https://user-images.githubusercontent.com/17949410/88499961-0ae86f00-cffa-11ea-83a2-c96d53063772.png)




 Comments: 
 Comment 0: Hello.

As far as I know that is merely a limitation of how a `tun` in the Linux kernel will report its speed. No matter the performance it will always report `10 Mb/s` but you should get much more out of it.

Have you tested the performance?

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