+void rmnet_vnd_setup(struct net_device *rmnet_dev)
+{
+	struct rmnet_priv *priv;
+
+	/* Clear out private data */
+	priv = netdev_priv(rmnet_dev);
+	memset(priv, 0, sizeof(struct rmnet_priv));

Netdev private area is always zero on creation. See alloc_netdev().

+struct rmnet_endpoint *rmnet_vnd_get_endpoint(struct net_device *rmnet_dev)
+{
+	struct rmnet_priv *priv;
+
+	if (!rmnet_dev)
+		return 0;

Do not confuse 0 with NULL. Did you run sparse?

+
+	priv = netdev_priv(rmnet_dev);
+	if (!priv)
+		return 0;

netdev_priv() always returns a non-NULL value.  The private area is just
a constant offset below the original network_device structure.