Currently, we only check the latest senders's exit code. If the receiver
report failed, it is not recoreded. Fix it by checking the exit code
of all the involved processes.

Before:
  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
 $ echo $?
 0

After:
  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
 $ echo $?
 1

Fixes: 3327a9c4 ("selftests: add functionals test for UDP GRO")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>