[selftest] udpgro test report fail but passed

[Hangbin Liu <liuhangbin@gmail.com>]

Hi Paolo,

In our recently internal testing, the udpgro.sh test reports failed but it
still return 0 as passed. e.g.

```
ipv6
 no GRO                                  ok
 no GRO chk cmsg                         ok
 GRO                                     ./udpgso_bench_rx: recv: bad packet len, got 1452, expected 14520

failed
 GRO chk cmsg                            ./udpgso_bench_rx: recv: bad packet len, got 1452, expected 14520

failed
 GRO with custom segment size            ./udpgso_bench_rx: recv: bad packet len, got 500, expected 14520

failed
 GRO with custom segment size cmsg       ./udpgso_bench_rx: recv: bad packet len, got 500, expected 14520

failed
 bad GRO lookup                          ok
 multiple GRO socks                      ./udpgso_bench_rx: recv: bad packet len, got 1452, expected 14520

./udpgso_bench_rx: recv: bad packet len, got 1452, expected 14520

failed
```

For run_one_2sock testing, I saw

```
        ip netns exec "${PEER_NS}" ./udpgso_bench_rx -C 1000 -R 10 ${rx_args} -p 12345 &
        ip netns exec "${PEER_NS}" ./udpgso_bench_rx -C 2000 -R 10 ${rx_args} && \
                echo "ok" || \
                echo "failed" &
        ...
        ./udpgso_bench_tx ${tx_args}
        ret=$?
        wait $(jobs -p)
        return $ret
```

So what's the effect if it echo "failed" while ret == 0?

Thanks
Hangbin