[IPV6] ROUTE: Don't try less preferred routes for on-link routes.

In addition to the real on-link routes, NONEXTHOP routes
should be considered on-link.

Problem reported by Meelis Roos <mroos@linux.ee>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: David S. Miller <davem@davemloft.net>