selftests: fixes for UDP GRO

The current implementation for UDP GRO tests is racy: the receiver
may flush the RX queue while the sending is still transmitting and
incorrectly report RX errors, with a wrong number of packet received.

Add explicit timeouts to the receiver for both connection activation
(first packet received for UDP) and reception completion, so that
in the above critical scenario the receiver will wait for the
transfer completion.

Fixes: 3327a9c4 ("selftests: add functionals test for UDP GRO")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>