mlxsw: spectrum_router: Allow more route types to be programmed

Allow 'unreachable', 'blackhole' and 'prohibit' route types to be
programmed into the device by sending any packet hitting them to the
CPU.

This is needed so that users will be able to program a default route
into the VRF's table, thereby preventing lookup from leaking to other
tables.

Audit the code paths to make sure we don't rely on the presence of a
nexthop netdev, as it doesn't exist for above mentioned route types.

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