ipv6: Change rt6_add_nexthop and rt6_nexthop_info to take fib6_nh

rt6_add_nexthop and rt6_nexthop_info only need the fib6_info for the
gateway flag and the nexthop weight, and the presence of a gateway is now
per-nexthop. Update the signatures to take a fib6_nh and nexthop weight
and better align with the ipv4 versions.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>