message: Fix length check in datalink's build_and_send()
  This is so build_and_send() can run successfully when num_packets *
  packet_size is equal to the write_buffer length.
diff: @@ -242,7 +242,7 @@ impl DataLinkSender for DataLinkSenderImpl {
  func: &mut dyn FnMut(&mut [u8]),
  ) -> Option<io::Result<()>> {
  let len = num_packets * packet_size;
  - if len < self.write_buffer.len() {
  + if len <= self.write_buffer.len() {
  let min = cmp::min(self.write_buffer[..].len(), len);
  let mut_slice = &mut self.write_buffer;
  for chunk in mut_slice[..min].chunks_mut(packet_size) {
  