mlxsw: spectrum_router: Add the nexthop neigh activity update

For nexthop neighbours we need to make kernel to think there is a traffic
flowing to them preventing it from going to stale state. Otherwise
kernel would stale it and eventually the neigh would be removed from HW
and nexthop as well. That would reduce ECMP group in HW.

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>