Function build_dhcp_request_payload 

fn build_dhcp_request_payload(
    xid: u32,
    mac: [u8; 6],
    src_port: u16,
    dst_port: u16,
    req_ip: [u8; 4],
    server: [u8; 4],
    out: &mut [u8],
) -> Result<usize, &'static str>