selftests/tcp_ao: Zero-init tcp_ao_info_opt

The structure is on the stack and has to be zero-initialized as
the kernel checks for:
>	if (in.reserved != 0 || in.reserved2 != 0)
>		return -EINVAL;

Fixes: b2666053 ("selftests/net: Add test for TCP-AO add setsockopt() command")
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>