icmpv6_filter() should not modify its input, or else its caller
would need to recompute ipv6_hdr() if skb->head is reallocated.

Use skb_header_pointer() instead of pskb_may_pull() and
change the prototype to make clear both sk and skb are const.

Also, if icmpv6 header cannot be found, do not deliver the packet,
as we do in IPv4.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>