ip_tunnels: new IP_TUNNEL_INFO_BRIDGE flag for ip_tunnel_info mode

New ip_tunnel_info flag to represent bridged tunnel metadata.
Used by bridge driver later in the series to pass per vlan dst
metadata to bridge ports.

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