selftests/net: relax cpu affinity requirement in msg_zerocopy test

The msg_zerocopy test pins the sender and receiver threads to separate
cores to reduce variance between runs.

But it hardcodes the cores and skips core 0, so it fails on machines
with the selected cores offline, or simply fewer cores.

The test mainly gives code coverage in automated runs. The throughput
of zerocopy ('-z') and non-zerocopy runs is logged for manual
inspection.

Continue even when sched_setaffinity fails. Just log to warn anyone
interpreting the data.

Fixes: 07b65c5b ("test: add msg_zerocopy test")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>