selftests: udpgro: report error when receive failed

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>