[netdevice] Reset network device index when last device is unregistered

When functioning as an EFI driver, drivers can be disconnected and
reconnected multiple times (e.g. via the EFI shell "connect" command,
or by running an executable such as ipxe.efi which will temporarily
disconnect existing drivers).

Minimise surprise by resetting the network device index to zero
whenever the last device is unregistered.  This is not foolproof, but
it does handle the common case of having all devices unregistered and
then reregistered in the original order.

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

src/net/netdevice.c

 /** List of open network devices, in reverse order of opening */
 static struct list_head open_net_devices = LIST_HEAD_INIT ( open_net_devices );
 
+/** Network device index */
+static unsigned int netdev_index = 0;
+
 /** Network polling profiler */
 static struct profiler net_poll_profiler __profiler = { .name = "net.poll" };
 
  * devices.
  */
 int register_netdev ( struct net_device *netdev ) {
-	static unsigned int ifindex = 0;
 	struct ll_protocol *ll_protocol = netdev->ll_protocol;
 	struct net_driver *driver;
 	uint32_t seed;
 	int rc;
 
 	/* Record device index and create device name */
-	netdev->index = ifindex++;
+	netdev->index = netdev_index++;
 	if ( netdev->name[0] == '\0' ) {
 		snprintf ( netdev->name, sizeof ( netdev->name ), "net%d",
 			   netdev->index );
 	DBGC ( netdev, "NETDEV %s unregistered\n", netdev->name );
 	list_del ( &netdev->list );
 	netdev_put ( netdev );
+
+	/* Reset network device index if no devices remain */
+	if ( list_empty ( &net_devices ) )
+		netdev_index = 0;
 }
 
 /** Enable or disable interrupts