bridge: uapi: add per vlan tunnel info

New nested netlink attribute to associate tunnel info per vlan.
This is used by bridge driver to send tunnel metadata to
bridge ports in vlan tunnel mode. This patch also adds new per
port flag IFLA_BRPORT_VLAN_TUNNEL to enable vlan tunnel mode.
off by default.

One example use for this is a vxlan bridging gateway or vtep
which maps vlans to vn-segments (or vnis). User can configure
per-vlan tunnel information which the bridge driver can use
to bridge vlan into the corresponding vn-segment.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>