mlxsw: spectrum_router: Add ability to overwrite adjacency entry only when inactive

Allow the driver to instruct the device to only overwrite an adjacency
entry if its activity is cleared. Currently, adjacency entry is always
overwritten, regardless of activity.

This will be used by subsequent patches to prevent replacement of active
nexthop buckets.

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