ip_gre: remove the incorrect mtu limit for ipgre tap

ip_gre: remove the incorrect mtu limit for ipgre tap

Upstream commit:
    commit cfddd4c33c254954927942599d299b3865743146
    Author: Xin Long <lucien.xin@gmail.com>
    Date:   Mon Dec 18 14:24:35 2017 +0800

    ip_gre: remove the incorrect mtu limit for ipgre tap

    ipgre tap driver calls ether_setup(), after commit 61e84623ace3
    ("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: 61e84623ace3 ("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>
Cc: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>