ipv6: l2tp: fix a potential issue in l2tp_ip6_recv

commit be447f305494e019dfc37ea4cdf3b0e4200b4eba upstream.

pskb_may_pull() can change skb->data, so we have to load ptr/optr at the
right place.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>