[IPV6] XFRM: Fix decoding session with preceding extension header(s).

We did not correctly decode session with preceding extension
header(s).  This was because we had already pulled preceding
headers, skb->nh.raw + 40 + 1 - skb->data was minus, and
pskb_may_pull() failed.

We now have IP6CB(skb)->nhoff and skb->h.raw, and we can
start parsing / decoding upper layer protocol from current
position.

Tracked down by Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
and tested by Kazunori Miyazawa <kazunori@miyazawa.org>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>