[IPV6]: Use skb->nh.ipv6h instead of casting skb->nh.raw

nh.ipv6h is there exactly for this reason! Use it while it exists ;-)

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>