mlxsw: spectrum_router: Allow programming link-local prefix routes

The device has a trap for IPv6 packets that need be routed and have a
unicast link-local destination IP (i.e., fe80::/10). This allows mlxsw
to ignore link-local routes, as the packets will be trapped to the CPU
in any case.

However, since link-local routes are not programmed, it is possible for
routed packets to hit the default route which might also be programmed
to trap packets. This means that packets with a link-local destination
IP might be trapped for the wrong reason.

To overcome this, allow programming link-local prefix routes (usually
one fe80::/64 per-table), so that the packets will be forwarded until
reaching the link-local trap.

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