ip_gre: remove the incorrect mtu limit for ipgre tap

ipgre tap driver calls ether_setup(), after commit 61e84623
("net: centralize net_device min/max MTU checking"), the range
of mtu is [min_mtu, max_mtu], which is [68, 1500] by default.

It causes the dev mtu of the ipgre tap device to not be greater
than 1500, this limit value is not correct for ipgre tap device.

Besides, it's .change_mtu already does the right check. So this
patch is just to set max_mtu as 0, and leave the check to it's
.change_mtu.

Fixes: 61e84623 ("net: centralize net_device min/max MTU checking")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1310,6 +1310,7 @@ static const struct net_device_ops erspan_netdev_ops = {
 static void ipgre_tap_setup(struct net_device *dev)
 {
 	ether_setup(dev);
+	dev->max_mtu = 0;
 	dev->netdev_ops	= &gre_tap_netdev_ops;
 	dev->priv_flags &= ~IFF_TX_SKB_SHARING;
 	dev->priv_flags	|= IFF_LIVE_ADDR_CHANGE;