mlxsw: spectrum_router: Offload neighbours based on NUD state change

Listen to any NEIGH_UPDATE events sent and program the device
accordingly. If NUD state is VALID and neighbour isn't yet offloaded,
then program it into the device's table. Otherwise, just edit its
parameters.

If NUD state machine transitioned neighbour out of VALID state and it's
present in the device's table, then remove it.

Note that the device is programmed in delayed work, as the netevent
notification chain is atomic and prevents us from going to sleep.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>