vxlan: Accept user specified MTU value when create new vxlan link

When create a new vxlan link, example:
  ip link add vtap mtu 1440 type vxlan vni 1 dev eth0

The argument "mtu" has no effect, because it is not set to conf->mtu. The
default value is used in vxlan_dev_configure function.

This problem was introduced by commit 0dfbdf41 (vxlan: Factor out device
configuration).

Fixes: 0dfbdf41 (vxlan: Factor out device configuration)
Signed-off-by: Chen Haiquan <oc@yunify.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>