selftests: correct define in msg_zerocopy.c

The msg_zerocopy test defines SO_ZEROCOPY if necessary, but its value
is inconsistent with the one in asm-generic.h. Correct that.

Also convert one error to a warning. When the test is complete, report
throughput and close cleanly even if the process did not wait for all
completions.

Reported-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>