[lotest] Discard packets arriving on the incorrect network device

Commit 24bbaf6 ("[lotest] Allow loopback testing on shared networks")
introduced a regression in which loopback testing packets would be
accepted from any network device.  This produces unexpected results,
such as VLAN loopback testing succeeding even when incorrectly using
the underlying trunk device as either transmitter or receiver.

Fix by discarding any loopback testing packets which arrive on a
network device other than the current loopback testing receiver.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/lotest.c

  *
  */
 
-/** Loopback testing in progress flag */
-static int lotest_active;
+/** Current loopback test receiver */
+static struct net_device *lotest_receiver;
 
 /** Loopback testing received packets */
 static LIST_HEAD ( lotest_queue );
  * @ret rc		Return status code
  */
 static int lotest_rx ( struct io_buffer *iobuf,
-		       struct net_device *netdev __unused,
+		       struct net_device *netdev,
 		       const void *ll_dest __unused,
 		       const void *ll_source __unused,
 		       unsigned int flags __unused ) {
 
 	/* Add to received packet queue if currently performing a test */
-	if ( lotest_active ) {
+	if ( netdev == lotest_receiver ) {
 		list_add_tail ( &iobuf->list, &lotest_queue );
 	} else {
 		free_iob ( iobuf );
 
 	/* Start loopback test */
 	lotest_flush();
-	lotest_active = 1;
+	lotest_receiver = receiver;
 
 	/* Perform loopback test */
 	for ( successes = 0 ; ; successes++ ) {
 	printf ( "\n");
 
 	/* Stop loopback testing */
-	lotest_active = 0;
+	lotest_receiver = NULL;
 	lotest_flush();
 
 	/* Dump final statistics */