mlxsw: spectrum_router: Replay neighbours when RIF is made

As neighbours are created, mlxsw is involved through the netevent
notifications. When at the time there is no RIF for a given neighbour, the
notification is not acted upon. When the RIF is later created, these
outstanding neighbours are left unoffloaded and cause traffic to go through
the SW datapath.

In order to fix this issue, as a RIF is created, walk the ARP and ND tables
and find neighbours for the netdevice that represents the RIF. Then
schedule neighbour work for them, allowing them to be offloaded.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>