[iobuf] Add iob_disown() and use it where it simplifies code

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

src/arch/i386/drivers/net/undinet.c

 					 undi_isr.Frame.offset, frag_len );
 			if ( iob_len ( iobuf ) == len ) {
 				/* Whole packet received; deliver it */
-				netdev_rx ( netdev, iobuf );
-				iobuf = NULL;
+				netdev_rx ( netdev, iob_disown ( iobuf ) );
 				/* Etherboot 5.4 fails to return all packets
 				 * under mild load; pretend it retriggered.
 				 */

src/include/gpxe/iobuf.h

 	iobuf->end = ( data + max_len );
 }
 
+/**
+ * Disown an I/O buffer
+ *
+ * @v iobuf	I/O buffer
+ *
+ * There are many functions that take ownership of the I/O buffer they
+ * are passed as a parameter.  The caller should not retain a pointer
+ * to the I/O buffer.  Use iob_disown() to automatically nullify the
+ * caller's pointer, e.g.:
+ *
+ *     xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );
+ *
+ * This will ensure that iobuf is set to NULL for any code after the
+ * call to xfer_deliver_iob().
+ */
+#define iob_disown( iobuf ) ( {				\
+	struct io_buffer *__iobuf = (iobuf);		\
+	(iobuf) = NULL;					\
+	__iobuf; } )
+
 extern struct io_buffer * __malloc alloc_iob ( size_t len );
 extern void free_iob ( struct io_buffer *iobuf );
 extern void iob_pad ( struct io_buffer *iobuf, size_t min_len );

src/interface/efi/efi_snp.c

 	}
 
 	/* Transmit packet */
-	if ( ( rc = netdev_tx ( snpdev->netdev, iobuf ) ) != 0 ) {
+	if ( ( rc = netdev_tx ( snpdev->netdev, iob_disown ( iobuf ) ) ) != 0){
 		DBGC ( snpdev, "SNPDEV %p TX could not transmit: %s\n",
 		       snpdev, strerror ( rc ) );
-		iobuf = NULL;
 		efirc = RC_TO_EFIRC ( rc );
 		goto err_tx;
 	}

src/net/arp.c

 	memcpy ( arp_sender_ha ( arphdr ), netdev->ll_addr, arphdr->ar_hln );
 
 	/* Send reply */
-	net_tx ( iobuf, netdev, &arp_protocol, arp_target_ha (arphdr ) );
-	iobuf = NULL;
+	net_tx ( iob_disown ( iobuf ), netdev, &arp_protocol,
+		 arp_target_ha ( arphdr ) );
 
  done:
 	free_iob ( iobuf );

src/net/tcp/http.c

 			/* Once we're into the data phase, just fill
 			 * the data buffer
 			 */
-			rc = http_rx_data ( http, iobuf );
-			iobuf = NULL;
+			rc = http_rx_data ( http, iob_disown ( iobuf ) );
 			goto done;
 		case HTTP_RX_RESPONSE:
 		case HTTP_RX_HEADER:

src/net/udp.c

 	memset ( &meta, 0, sizeof ( meta ) );
 	meta.src = ( struct sockaddr * ) st_src;
 	meta.dest = ( struct sockaddr * ) st_dest;
-	rc = xfer_deliver_iob_meta ( &udp->xfer, iobuf, &meta );
-	iobuf = NULL;
+	rc = xfer_deliver_iob_meta ( &udp->xfer, iob_disown ( iobuf ), &meta );
 
  done:
 	free_iob ( iobuf );

src/net/udp/dhcp.c

 
 	/* Transmit the packet */
 	iob_put ( iobuf, dhcppkt.len );
-	rc = xfer_deliver_iob_meta ( &dhcp->xfer, iobuf, &meta );
-	iobuf = NULL;
-	if ( rc != 0 ) {
+	if ( ( rc = xfer_deliver_iob_meta ( &dhcp->xfer, iob_disown ( iobuf ),
+					    &meta ) ) != 0 ) {
 		DBGC ( dhcp, "DHCP %p could not transmit UDP packet: %s\n",
 		       dhcp, strerror ( rc ) );
 		goto done;

src/net/udp/tftp.c

 	memset ( &meta, 0, sizeof ( meta ) );
 	meta.whence = SEEK_SET;
 	meta.offset = offset;
-	rc = xfer_deliver_iob_meta ( &tftp->xfer, iobuf, &meta );
-	iobuf = NULL;
-	if ( rc != 0 ) {
+	if ( ( rc = xfer_deliver_iob_meta ( &tftp->xfer, iob_disown ( iobuf ),
+					    &meta ) ) != 0 ) {
 		DBGC ( tftp, "TFTP %p could not deliver data: %s\n",
 		       tftp, strerror ( rc ) );
 		goto done;
 		rc = tftp_rx_oack ( tftp, iobuf->data, len );
 		break;
 	case htons ( TFTP_DATA ):
-		rc = tftp_rx_data ( tftp, iobuf );
-		iobuf = NULL;
+		rc = tftp_rx_data ( tftp, iob_disown ( iobuf ) );
 		break;
 	case htons ( TFTP_ERROR ):
 		rc = tftp_rx_error ( tftp, iobuf->data, len );