ip6_gre: remove the incorrect mtu limit for ipgre tap

The same fix as the patch "ip_gre: remove the incorrect mtu limit for
ipgre tap" is also needed for ip6_gre.

Fixes: 61e84623 ("net: centralize net_device min/max MTU checking")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1308,6 +1308,7 @@ static void ip6gre_tap_setup(struct net_device *dev)
 
 	ether_setup(dev);
 
+	dev->max_mtu = 0;
 	dev->netdev_ops = &ip6gre_tap_netdev_ops;
 	dev->needs_free_netdev = true;
 	dev->priv_destructor = ip6gre_dev_free;