[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/net/tcp/ftp.c

@@ -380,7 +380,7 @@ static void ftp_data_closed ( struct xfer_interface *data, int rc ) {
  *
  * @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

@@ -321,7 +321,7 @@ static int http_rx_data ( struct http_request *http,
  *
  * @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,