ipv6: fix a potential use after free in sit.c

pskb_may_pull() maybe change skb->data and make iph pointer oboslete,
fix it by geting ip header length directly.

Fixes: ca15a078 (sit: generate icmpv6 error when receiving icmpv4 error)
Cc: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>