batman-adv: Disallow regular interface as mesh device

When trying to associate a net_device with another net_device which
already exists, batman-adv assumes that this interface is a fully
initialized batman mesh interface without checking it. The behaviour
when accessing data behind netdev_priv of a random net_device is
undefined and potentially dangerous.

Reported-by: Linus Lüssing <linus.luessing@ascom.ch>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
diff --git a/net/batman-adv/soft-interface.h b/net/batman-adv/soft-interface.h
index 80a3607..4789b6f 100644
--- a/net/batman-adv/soft-interface.h
+++ b/net/batman-adv/soft-interface.h
@@ -31,5 +31,6 @@
 		  int hdr_size);
 struct net_device *softif_create(char *name);
 void softif_destroy(struct net_device *soft_iface);
+int softif_is_valid(struct net_device *net_dev);
 
 #endif /* _NET_BATMAN_ADV_SOFT_INTERFACE_H_ */