ipv6: add ip6_route_lookup

like rt6_lookup, but allows caller to pass in flowi6 structure.
Will be used by the upcoming ipv6 netfilter reverse path filter
match.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>