ipv6: Pass fib6_nh and flags to rt6_score_route

rt6_score_route only needs the fib6_flags and nexthop data. Change
it accordingly. Allows re-use later for nexthop based fib6_nh.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>