If we don't have IPv6 default routes, we assume all ipv6 destinations 
are on-link as specified in RFC2461. It, however, is considered harmful now;
it is problematic with IPv6-capable nodes that do not have off-link
IPv6 connectivity (eg no default routers) and such nodes will take
a few seconds until they fall back to use IPv4.

See <draft-ietf-v6ops-onlinkassumption-02.txt> for details.

From: KUNITAKE Koichi <kunitake@anchor.jp>
Signed-off-by: KUNITAKE Koichi <kunitake@anchor.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>