ipv6: Introduce rt6_nexthop() to select nexthop address.

For RTF_GATEWAY route, return rt->rt6i_gateway.
Otherwise, return 2nd argument (destination address).

This will be used by following patches which remove rt->n
dependency patches in ip6_dst_lookup_tail() and ip6_finish_output2().

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