selftests/net: make udpgso_bench skip unsupported testcases

Kselftest can be run against older kernels. Instead of failing hard
when a feature is unsupported, return the KSFT_SKIP exit code.

Specifically, do not fail hard on missing udp zerocopy.

The udp gso bench test runs multiple test cases from a single script.
Fail if any case fails, else return skip if any test is skipped.

Link: https://lore.kernel.org/lkml/20190618171516.GA17547@kroah.com/


Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>