ipv6: fix a potential use after free in ip6_offload.c

pskb_may_pull() maybe change skb->data and make opth pointer oboslete,
so set the opth again

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>