mlxsw: spectrum_router: Offload blackhole routes

Create a new FIB entry type for blackhole routes and set it in case the
type of the notified route is 'RTN_BLACKHOLE'.

Program such routes with a discard action and mark them as offloaded
since the device is dropping the packets instead of the kernel.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>