[lotest] Include sequence number within loopback test packets

Include a sequence number as the first four bytes of the loopback test
packet payload.  When a content mismatch occurs, this gives some
information about the source of the mismatched packet.

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

src/usr/lotest.c

  */
 int loopback_test ( struct net_device *sender, struct net_device *receiver,
 		    size_t mtu ) {
-	uint8_t buf[mtu];
+	uint8_t *buf;
+	uint32_t *seq;
 	struct io_buffer *iobuf;
 	unsigned int i;
 	unsigned int successes;
 	if ( ( rc = iflinkwait ( receiver, LINK_WAIT_MS ) ) != 0 )
 		return rc;
 
+	/* Allocate data buffer */
+	if ( mtu < sizeof ( *seq ) )
+		mtu = sizeof ( *seq );
+	buf = malloc ( mtu );
+	if ( ! buf )
+		return -ENOMEM;
+	seq = ( ( void * ) buf );
+
 	/* Print initial statistics */
 	printf ( "Performing loopback test from %s to %s with %zd byte MTU\n",
 		 sender->name, receiver->name, mtu );
 		printf ( "\r%d", successes );
 
 		/* Generate random packet */
-		for ( i = 0 ; i < sizeof ( buf ) ; i++ )
+		*seq = htonl ( successes );
+		for ( i = sizeof ( *seq ) ; i < mtu ; i++ )
 			buf[i] = random();
-		iobuf = alloc_iob ( MAX_LL_HEADER_LEN + sizeof ( buf ) );
+		iobuf = alloc_iob ( MAX_LL_HEADER_LEN + mtu );
 		if ( ! iobuf ) {
 			printf ( "\nFailed to allocate I/O buffer" );
 			rc = -ENOMEM;
 			break;
 		}
 		iob_reserve ( iobuf, MAX_LL_HEADER_LEN );
-		memcpy ( iob_put ( iobuf, sizeof ( buf ) ),
-			 buf, sizeof ( buf ) );
+		memcpy ( iob_put ( iobuf, mtu ), buf, mtu );
 
 		/* Transmit packet */
 		if ( ( rc = net_tx ( iob_disown ( iobuf ), sender,
 		}
 
 		/* Wait for received packet */
-		if ( ( rc = loopback_wait ( receiver, buf,
-					    sizeof ( buf ) ) ) != 0 ) {
+		if ( ( rc = loopback_wait ( receiver, buf, mtu ) ) != 0 )
 			break;
-		}
 	}
 
 	printf ( "\n");
 	ifstat ( sender );
 	ifstat ( receiver );
 
+	/* Free buffer */
+	free ( buf );
+
 	return 0;
 }