net: vrf: remove MTU limits for vrf device

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>