can: export a safe netdev_priv wrapper for candev

In net_device notifier calls, it was impossible to determine
if a CAN device is based on candev in a safe way.
This patch adds such test in order to access candev storage
from within those notifiers.

Signed-off-by: Kurt Van Dijck <redacted>

diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
index 963e2cc..6c1e704 100644
--- a/drivers/net/can/dev.c
+++ b/drivers/net/can/dev.c

@@ -795,6 +795,18 @@ void unregister_candev(struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(unregister_candev);
 
+/*
+ * Test if a network device is a candev based device
+ * and return the can_priv* if so.
+ */
+struct can_priv *safe_candev_priv(struct net_device *dev)
+{
+	if ((dev->type != ARPHRD_CAN) || (dev->rtnl_link_ops != &can_link_ops))
+		return NULL;
+
+	return netdev_priv(dev);
+}
+