openvswitch: Make metadata_dst tunnel work in IP_TUNNEL_INFO_BRIDGE mode

There is currently no support for the multicast/broadcast aspects
of VXLAN in ovs. In the datapath flow the tun_dst must specific.
But in the IP_TUNNEL_INFO_BRIDGE mode the tun_dst can not be specific.
And the packet can forward through the fdb table of vxlan devcice. In
this mode the broadcast/multicast packet can be sent through the
following ways in ovs.

ovs-vsctl add-port br0 vxlan -- set in vxlan type=vxlan \
        options:key=1000 options:remote_ip=flow
ovs-ofctl add-flow br0 in_port=LOCAL,dl_dst=ff:ff:ff:ff:ff:ff, \
        action=output:vxlan

bridge fdb append ff:ff:ff:ff:ff:ff dev vxlan_sys_4789 dst 172.168.0.1 \
        src_vni 1000 vni 1000 self
bridge fdb append ff:ff:ff:ff:ff:ff dev vxlan_sys_4789 dst 172.168.0.2 \
        src_vni 1000 vni 1000 self

Signed-off-by: wenxu <wenxu@ucloud.cn>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>