Occasionally unable to route over l2tp0

[eenblam]

@paidforby observed this in #148 (comment)

Looking at the exitnode logs, the tunnel to your node was established and then closed 2 minutes later, and this happened several times in a row.

The broker error 3 is returned, indicating that the broker hasn't heard from the client in 2 minutes.

While debugging the Benvenue site, I observed the same bug from the client side. I was receiving routes from Babel over UDP, but I couldn't send any packets over l2tp0 whatsoever (rnetlink answers - address unreachable). No idea why this was happening, but a couple of reboots magically fixed it.