[IPV6]: Fix xfrm6_tunnel_check_size mtu calc.

OK, let's make it the same as IPv4.  This is still broken
as the calculation is wrong.  However, to fix it properly
we need to store the MTUs inside the dst's.  I'll get
around to that sooner or later.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/xfrm6_output.c

@@ -79,7 +79,7 @@ static int xfrm6_tunnel_check_size(struct sk_buff *skb)
 	int mtu, ret = 0;
 	struct dst_entry *dst = skb->dst;
 
-	mtu = dst_pmtu(dst) - sizeof(struct ipv6hdr);
+	mtu = dst_pmtu(dst) - dst->header_len - dst->trailer_len;
 	if (mtu < IPV6_MIN_MTU)
 		mtu = IPV6_MIN_MTU;