[IPV6]: ROUTE: More strict check for default routers in rt6_get_dflt_router().

Check RTF_ADDRCONF|RTF_DEFAULT in rt6_get_dflt_router().

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