ip6_gre: remove the incorrect mtu limit for ipgre tap

[ Upstream commit 2c52129a7d74d017320804c6928de770815c5f4a ]

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: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>