ipv4: l2tp: fix a potential issue in l2tp_ip_recv

commit 5745b8232e942abd5e16e85fa9b27cc21324acf0 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>