This patch adds support for bridge vlan_protocol.

Example:
$ ip link set br0 type bridge vlan_protocol 802.1ad
$ ip -d link show br0
4: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
UP mode DEFAULT group default qlen 1000
    link/ether 44:37:e6:ab:cd:ef brd ff:ff:ff:ff:ff:ff promiscuity 0
    bridge forward_delay 0 hello_time 200 max_age 2000 ageing_time 30000
stp_state 0 priority 32768 vlan_filtering 0 vlan_protocol 802.1ad
addrgenmode eui64
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>