batman-adv: warn if added interface is part of a bridge

	if (!atomic_inc_not_zero(&hard_iface->refcount))

		goto out;

	/* hard-interface is part of a bridge */

	if (hard_iface->net_dev->priv_flags & IFF_BRIDGE_PORT)

		pr_err("You are about to enable batman-adv on '%s' which "

		       "already is part of a bridge. Unless you know exactly "

		       "what you are doing this is probably wrong and won't "

		       "work the way you think it would.\n",

		       hard_iface->net_dev->name);

	soft_iface = dev_get_by_name(&init_net, iface_name);

	if (!soft_iface) {