lib/tun: Add 'void *priv' pointer to tun_t

This allows the application to attach some private state to the tun
device, such as the context from which it was created/allocated

Change-Id: Ief43b9b5fab5830fa8e28362c795f88f0b4d353b

lib/tun.h

@@ -61,6 +61,8 @@ struct tun_t {
 	int routes;		/* One if we allocated an automatic route */
 	char devname[IFNAMSIZ];	/* Name of the tun device */
 	int (*cb_ind) (struct tun_t * tun, void *pack, unsigned len);
+	/* to be used by libgtp callers/users (to attach their own private state) */
+	void *priv;
 };
 
 extern int tun_new(struct tun_t **tun);