selftests/net: synchronize udpgro tests' tx and rx connection

The sockets used by udpgso_bench_tx aren't always ready when
udpgso_bench_tx transmits packets. This issue is more prevalent in -rt
kernels, but can occur in both. Replace the hacky sleep calls with a
function that checks whether the ports in the namespace are ready for
use.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Lucas Karpinski <lkarpins@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>