fix: ptp deadlock on send

[samuelemusiani]

I have noticed that if you connect two vdens using the ptp plugin, the basic functionalities of the network would work (es. ping), but as soon as you start iperf3 the connection would stop working. I have observed that if you use the --bidir flag on iperf3 and your system is under stress (maybe using stress-ng) the bug is more easily reproducible.

I have found that the bug was in the ptp plugin. In particular there is very specific case in which both ends of the connection would write to the socket used for the connection, but the socket is full (unix sockets on Linux have a max capacity, on my system is around 95000 bytes). This would cause a deadlock as both send would block.

I fixed this by adding the MSG_DONTWAIT flag that enables nonblocking operation.

Let me know if this makes sens.