MINOR: raw_sock: directly call fd_stop_send() and not conn_xprt_stop_send()

Now that we know that the connection layer is transparent for polling
changes, we have no reason for hiding behind conn_xprt_stop_send() and
can safely call fd_stop_send() on the FD once the buffer is empty.

src/raw_sock.c

@@ -382,7 +382,7 @@ static size_t raw_sock_from_buf(struct connection *conn, void *xprt_ctx, const s
 			if (ret < try)
 				break;
 			if (!count)
-				conn_xprt_stop_send(conn);
+				fd_stop_send(conn->handle.fd);
 		}
 		else if (ret == 0 || errno == EAGAIN || errno == ENOTCONN || errno == EINPROGRESS) {
 			/* nothing written, we need to poll for write first */