nfp: flower vxlan neighbour keep-alive

Periodically receive messages containing the destination IPs of tunnels
that have recently forwarded traffic. Update the neighbour entries 'used'
value for these IPs next hop.

This prevents the neighbour entry from expiring on timeout but rather
signals an ARP to verify the connection. From an NFP perspective, packets
will not fall back mid-flow unless the link is verified to be down.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>