test/vsock: MSG_ZEROCOPY support for vsock_perf

To use this option pass '--zerocopy' parameter:

./vsock_perf --zerocopy --sender <cid> ...

With this option MSG_ZEROCOPY flag will be passed to the 'send()' call.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>