iplink: bridge: support bridge port vlan_tunnel attribute

iplink: bridge: support bridge port vlan_tunnel attribute

This config maps to IFLA_BRPORT_VLAN_TUNNEL bridge port netlink
flag attribute. This flag enables vlan to tunnel mapping on a bridge
port. It is off by default.

set vlan_tunnel attribute on bridge port vxlan0:

$ip link set dev vxlan0 type bridge_slave vlan_tunnel on
$ip link set dev vxlan0 type bridge_slave vlan_tunnel off

or via bridge command

$bridge link set dev vxlan0 vlan_tunnel on
$bridge link set dev vxlan0 vlan_tunnel off

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>