can: netlink: dump bitrate 0 if can_priv::bittiming.bitrate is -1U

Upcoming changes on slcan driver will require you to specify a bitrate
of value -1 to prevent the open_candev() from failing but at the same
time highlighting that it is a fake value. In this case the command
`ip --details -s -s link show' would print 4294967295 as the bitrate
value. The patch change this value in 0.

Link: https://lore.kernel.org/all/20220628163137.413025-5-dario.binacchi@amarulasolutions.com
Suggested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Tested-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev/netlink.c

@@ -511,7 +511,8 @@ static int can_fill_info(struct sk_buff *skb, const struct net_device *dev)
 	if (priv->do_get_state)
 		priv->do_get_state(dev, &state);
 
-	if ((priv->bittiming.bitrate &&
+	if ((priv->bittiming.bitrate != CAN_BITRATE_UNSET &&
+	     priv->bittiming.bitrate != CAN_BITRATE_UNKNOWN &&
 	     nla_put(skb, IFLA_CAN_BITTIMING,
 		     sizeof(priv->bittiming), &priv->bittiming)) ||
 

include/linux/can/bittiming.h

@@ -11,6 +11,8 @@
 
 #define CAN_SYNC_SEG 1
 
+#define CAN_BITRATE_UNSET 0
+#define CAN_BITRATE_UNKNOWN (-1U)
 
 #define CAN_CTRLMODE_TDC_MASK					\
 	(CAN_CTRLMODE_TDC_AUTO | CAN_CTRLMODE_TDC_MANUAL)