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

[ Upstream commit 9ef2e965e55481a52d6d91ce61977a27836268d3 ]

This is a clone of commit 2ab957492d13b ("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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>