[IPV6] Make ipsec tunnel work with ext hdrs

I noticed that using extensions headers along with IPsecv6 tunnel mode did
not work in 2.5.73 + patch-2.5.73-bk3. The following patch
checks "nexthdr" instead of "iph->nexthdr" which could be an
extension header. I tested this with tunnel mode and transport mode
with and without extension headers and it worked ok.

net/ipv6/xfrm6_input.c

@@ -67,10 +67,8 @@ int xfrm6_rcv(struct sk_buff **pskb, unsigned int *nhoffp)
 
 		xfrm_vec[xfrm_nr++].xvec = x;
 
-		iph = skb->nh.ipv6h;
-
 		if (x->props.mode) { /* XXX */
-			if (iph->nexthdr != IPPROTO_IPV6)
+			if (nexthdr != IPPROTO_IPV6)
 				goto drop;
 			skb->nh.raw = skb->data;
 			iph = skb->nh.ipv6h;