net: vrf: remove MTU limits for vrf device

mainline inclusion
from mainline-5.0
commit ad49bc63
category: bugfix
bugzilla: 10915
CVE: NA

-------------------------------------------------

Similiar to commit e94cd811 ("net: remove MTU limits for dummy and
ifb device"), MTU is irrelevant for VRF device. We init it as 64K while
limit it to [68, 1500] may make users feel confused.

Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Reviewed-by: Wenan Mao <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>