ip_gre: remove redundant variables t_hlen

After commit ffc2b6ee ("ip_gre: fix IFLA_MTU ignored on NEWLINK")
variable t_hlen is assigned values that are never read,
hence they are redundant and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>