mlxsw: spectrum_router: Periodically update activity of nexthop buckets

The kernel periodically checks the idle time of nexthop buckets to
determine if they are idle and can be re-populated with a new nexthop.

When the resilient nexthop group is offloaded to hardware, the kernel
will not see activity on nexthop buckets unless it is reported from
hardware.

Therefore, periodically (every 1 second) query the hardware for activity
of adjacency entries used as part of a resilient nexthop group and
report it to the nexthop code.

The activity is only queried if resilient nexthop groups are in use. The
delayed work is canceled otherwise.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>