socket: Used the unfilled section properly

    ) -> Poll<std::io::Result<()>> {

        self.with(|s| match s.can_recv() {

            true => {

                let rbuf = s.recv(|b| (buf.remaining(), b));

                let rbuf = s.recv_slice(buf.initialize_unfilled());

                match rbuf {

                    Err(e) => {

                        return Poll::Ready(Err(std::io::Error::new(

                            format!("{}", e),

                        )))

                    }

                    Ok(b) => {

                        if b.len() > 0 {

                            buf.put_slice(b);

                    Ok(n) => {

                        if n > 0 {

                            buf.set_filled(n);

                            Poll::Ready(Ok(()))

                        } else {

                            s.register_recv_waker(cx.waker());