ipv6: use rt6_get_dflt_router to get default router in rt6_route_rcv

As the rfc 4191 said, the Router Preference and Lifetime values in a
::/0 Route Information Option should override the preference and lifetime
values in the Router Advertisement header. But when the kernel deals with
a ::/0 Route Information Option, the rt6_get_route_info() always return
NULL, that means that overriding will not happen, because those default
routers were added without flag RTF_ROUTEINFO in rt6_add_dflt_router().

In order to deal with that condition, we should call rt6_get_dflt_router
when the prefix length is 0.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>