__ip_tunnel_change_mtu
int __ip_tunnel_change_mtu(struct net_device *dev, int new_mtu, bool strict);
err = __ip_tunnel_change_mtu(dev, IP_MAX_MTU, false);
EXPORT_SYMBOL_GPL(__ip_tunnel_change_mtu);
return __ip_tunnel_change_mtu(dev, new_mtu, true);