net: bridge: vlan options: nest the tunnel id into a tunnel info attribute

While discussing the new API, Roopa mentioned that we'll be adding more
tunnel attributes and options in the future, so it's better to make it a
nested attribute, since this is still in net-next we can easily change it
and nest the tunnel id attribute under BRIDGE_VLANDB_ENTRY_TUNNEL_INFO.

The new format is:
 [BRIDGE_VLANDB_ENTRY]
     [BRIDGE_VLANDB_ENTRY_TUNNEL_INFO]
         [BRIDGE_VLANDB_TINFO_ID]

Any new tunnel attributes can be nested under
BRIDGE_VLANDB_ENTRY_TUNNEL_INFO.

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