ip_tunnels: Add nexthop ID field to ip_tunnel_key

Extend the ip_tunnel_key structure with a field indicating the ID of the
nexthop object via which the skb should be routed.

The field is going to be populated in subsequent patches by the bridge
driver in order to indicate to the VXLAN driver which FDB nexthop object
to use in order to reach the target host.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>