selftests: fix the return value for UDP GRO test

The udpgro.sh will always return 0 (unless the bpf selftest was not
build first) even if there are some failed sub test-cases.

Therefore the kselftest framework will report this case is OK.

Check and return the exit status of each test to make it easier to
spot real failures.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>