Incorrect computation of the MTU in wg-quick

[Jason A. Donenfeld]

> I would like to report a behavior that seems to be incorrect in the way
> wg-quick computes the MTU to assign to a wireguard interface:
> https://git.zx2c4.com/wireguard-tools/tree/src/wg-quick/linux.bash#n125
> In this block, wg-quick goes through every endpoint it knows about,
> gets the mtu of the route to reach the endpoint, and takes the highest
> value among all the computed values.
> However it appears to me that the chosen value should instead be the
> lowest among all endpoints rather than the highest.
> 
> As an example, if I declare myself (localhost) as an endpoint (it may or
> may not be supported, but that’s how I found about this issue), then the
> mtu will be set to 65456 (65536-80) which is higher than what the other
> endpoints are able to manage, and I’ll only be able to contact myself
> properly.

Thanks for the report. Fixed:
https://git.zx2c4.com/wireguard-tools/commit/?id=5150cd647073be1f1c12688aef291bdf17970154

Let me know if that looks okay to you.

Jason