ip6_finish_output2 change broke netfilter xt_TEE target

The change made in commit 6fd6ce20 (ipv6: Do not depend on rt->n in
ip6_finish_output2) broke the xt_TEE target for IPv6 packets.  Instead
of using the nexthop provided in the --gateway option, ip6_finish_output2
is now performing neighbor solicitation for the original daddr in the
copied skb.

Similar breakage occurred in IPv4, and was fixed (in 2ad5b9e4) by using
the flag FLOWI_FLAG_KNOWN_NH.  I can find no easy way to make use of that
flag here.  Reverting 6fd6ce20 makes TEE work again, but I am not clear
on what problem that commit was attempting to solve.  Yoshifuji?

Phil