vlan: add MTU validation to loadNetConf

[mars1024]

Try to fix #393

Same like #404 , vlan plugin has the same problem if we want to create a vlan interface from a parent(master) interface with greater MTU.

[root@test ~]# ip link show eth2
11: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether fa:f2:3b:5a:33:02 brd ff:ff:ff:ff:ff:ff
[root@test ~]# ip link add link eth2 name eth2.100 mtu 2000 type vlan id 100
RTNETLINK answers: Invalid argument
[root@test ~]# ip link add link eth2 name eth2.100 mtu 1200 type vlan id 100
[root@test ~]# ip link show eth2.100
15: eth2.100@eth2: <BROADCAST,MULTICAST> mtu 1200 qdisc noop state DOWN mode DEFAULT group default qlen 1000
link/ether fa:f2:3b:5a:33:02 brd ff:ff:ff:ff:ff:ff

[dcbw]

/lgtm

[squeed]

👍