[xfer] Make consistent assumptions that xfer metadata can never be NULL

The documentation in xfer.h and xfer.c does not say that the metadata
parameter is optional in calls such as xfer_deliver_iob_meta() and the
deliver_iob() method.  However, some code in net/ is prepared to
accept a NULL pointer, and xfer_deliver_as_iob() passes a NULL pointer
directly to the deliver_iob() method.

Fix this mess of conflicting assumptions by making everything assume
that the metadata parameter is mandatory, and fixing
xfer_deliver_as_iob() to pass in a dummy metadata structure (as is
already done in xfer_deliver_iob()).

src/core/xfer.c

  *
  */
 
+/**
+ * Dummy transfer metadata
+ *
+ * This gets passed to xfer_interface::deliver_iob() and equivalents
+ * when no metadata is available.
+ */
+static struct xfer_metadata dummy_metadata;
+
 /**
  * Close data transfer interface
  *
  */
 int xfer_deliver_iob ( struct xfer_interface *xfer,
 		       struct io_buffer *iobuf ) {
-	static struct xfer_metadata dummy_metadata;
 	return xfer_deliver_iob_meta ( xfer, iobuf, &dummy_metadata );
 }
 
 		return -ENOMEM;
 
 	memcpy ( iob_put ( iobuf, len ), data, len );
-	return xfer->op->deliver_iob ( xfer, iobuf, NULL );
+	return xfer->op->deliver_iob ( xfer, iobuf, &dummy_metadata );
 }
 
 /**

src/net/tcp.c

  *
  * @v xfer		Data transfer interface
  * @v iobuf		Datagram I/O buffer
- * @v meta		Data transfer metadata, or NULL
+ * @v meta		Data transfer metadata
  * @ret rc		Return status code
  */
 static int tcp_xfer_deliver_iob ( struct xfer_interface *xfer,

src/net/tcp/ftp.c

  *
  * @v xfer		FTP data channel interface
  * @v iobuf		I/O buffer
- * @v meta		Data transfer metadata, or NULL
+ * @v meta		Data transfer metadata
  * @ret rc		Return status code
  */
 static int ftp_data_deliver_iob ( struct xfer_interface *data,

src/net/tcp/http.c

  *
  * @v socket		Transport layer interface
  * @v iobuf		I/O buffer
- * @v meta		Data transfer metadata, or NULL
+ * @v meta		Data transfer metadata
  * @ret rc		Return status code
  */
 static int http_socket_deliver_iob ( struct xfer_interface *socket,

src/net/udp.c

  *
  * @v xfer		Data transfer interface
  * @v iobuf		Datagram I/O buffer
- * @v meta		Data transfer metadata, or NULL
+ * @v meta		Data transfer metadata
  * @ret rc		Return status code
  */
 static int udp_xfer_deliver_iob ( struct xfer_interface *xfer,
 				  struct xfer_metadata *meta ) {
 	struct udp_connection *udp =
 		container_of ( xfer, struct udp_connection, xfer );
-	struct sockaddr_tcpip *src = NULL;
-	struct sockaddr_tcpip *dest = NULL;
-	struct net_device *netdev = NULL;
-
-	/* Apply xfer metadata */
-	if ( meta ) {
-		src = ( struct sockaddr_tcpip * ) meta->src;
-		dest = ( struct sockaddr_tcpip * ) meta->dest;
-		netdev = meta->netdev;
-	}
 
 	/* Transmit data, if possible */
-	udp_tx ( udp, iobuf, src, dest, netdev );
+	udp_tx ( udp, iobuf, ( ( struct sockaddr_tcpip * ) meta->src ),
+		 ( ( struct sockaddr_tcpip * ) meta->dest ), meta->netdev );
 
 	return 0;
 }

src/net/udp/dhcp.c

 	int rc = 0;
 
 	/* Sanity checks */
-	if ( ! meta ) {
-		DBGC ( dhcp, "DHCP %p received packet without metadata\n",
-		       dhcp );
-		rc = -EINVAL;
-		goto err_no_meta;
-	}
 	if ( ! meta->src ) {
 		DBGC ( dhcp, "DHCP %p received packet without source port\n",
 		       dhcp );
 	dhcppkt_put ( dhcppkt );
  err_alloc_dhcppkt:
  err_no_src:
- err_no_meta:
 	free_iob ( iobuf );
 	return rc;
 }

src/net/udp/tftp.c

  *
  * @v tftp		TFTP connection
  * @v iobuf		I/O buffer
- * @v meta		Transfer metadata, or NULL
+ * @v meta		Transfer metadata
  * @ret rc		Return status code
  */
 static int tftp_rx ( struct tftp_request *tftp,
 		       "%zd\n", tftp, len );
 		goto done;
 	}
-	if ( ! meta ) {
-		DBGC ( tftp, "TFTP %p received packet without metadata\n",
-		       tftp );
-		goto done;
-	}
 	if ( ! meta->src ) {
 		DBGC ( tftp, "TFTP %p received packet without source port\n",
 		       tftp );
  *
  * @v socket		Transport layer interface
  * @v iobuf		I/O buffer
- * @v meta		Transfer metadata, or NULL
+ * @v meta		Transfer metadata
  * @ret rc		Return status code
  */
 static int tftp_socket_deliver_iob ( struct xfer_interface *socket,
  *
  * @v mc_socket		Multicast transport layer interface
  * @v iobuf		I/O buffer
- * @v meta		Transfer metadata, or NULL
+ * @v meta		Transfer metadata
  * @ret rc		Return status code
  */
 static int tftp_mc_socket_deliver_iob ( struct xfer_interface *mc_socket,