mlxsw: spectrum_router: Add nexthop bucket replacement support

Replace a single nexthop bucket upon receiving a
'NEXTHOP_EVENT_BUCKET_REPLACE' notification.

When the 'force' parameter is not set, instruct the device to only
overwrite an adjacency entry if its activity is cleared, so as not to
break existing flows using the adjacency entry. The device does not
provide feedback if the replacement was successful in this case, so the
contents of the adjacency entry after the replacement are compared with
the replacement request.

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