If we have two sockets and send an upd message from one to another,
we can't be sure that it will be delivered immediately,
there is a change to get from recv(..., MSG_DONTWAIT) EGAIN.

This message is handled asynchronously in a kernel space, so
there may be a small timeout before we get it from another socket.

v2: fix socket6_udp too

Reported-by: Mr Travis
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>