net: bridge: add vlan_tunnel to bridge port policies

Found another missing port flag policy entry for IFLA_BRPORT_VLAN_TUNNEL
so add it now.

CC: Roopa Prabhu <roopa@cumulusnetworks.com>
Fixes: efa5356b ("bridge: per vlan dst_metadata netlink support")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>