netfilter: use skb_to_full_sk in ip6_route_me_harder

For some reason, Florian forgot to apply to ip6_route_me_harder
the fix that went in commit 29e09229 ("netfilter: use
skb_to_full_sk in ip_route_me_harder")

Fixes: ca6fb065

 ("tcp: attach SYNACK messages to request sockets instead of listener") 
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>