ip6gre_err() miscomputes grehlen (sizeof(ipv6h) is 4 or 8,
not 40 as expected), and should take into account 'offset' parameter.

Also uses pskb_may_pull() to cope with some fragged skbs
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Dmitry Kozlov <xeb@mail.ru>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>