ipv6: drop frames with attached skb->sk in forwarding

This is a clone of commit 2ab95749 ("ip_forward: Drop frames with
attached skb->sk") for ipv6.

This commit has exactly the same reasons as the above mentioned commit,
namely to prevent panics during netfilter reload or a misconfigured stack.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>