This patch implements support for the IFLA_BR_VLAN_FILTERING attribute
in iproute2 so it can enable/disable vlan_filtering.

Example:
$ ip link set br0 type bridge vlan_filtering 1
$ ip -d link show br0
6: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
UP mode DEFAULT group default
    link/ether 08:00:27:ea:07:38 brd ff:ff:ff:ff:ff:ff promiscuity 0
    bridge forward_delay 1500 hello_time 200 max_age 2000 vlan_filtering 1
    addrgenmode eui64
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>