ip_gre: include route header_len in max_headroom calculation

Taking route's header_len into account, and updating gre device
needed_headroom will give better hints on upper bound of required
headroom. This is useful if the gre traffic is xfrm'ed.

Signed-off-by: Timo Teras <timo.teras@iki.fi>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -810,11 +810,13 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
  tunnel->err_count = 0;
  }
 
- max_headroom = LL_RESERVED_SPACE(tdev) + gre_hlen;
+ max_headroom = LL_RESERVED_SPACE(tdev) + gre_hlen + rt->u.dst.header_len;
 
  if (skb_headroom(skb) < max_headroom || skb_shared(skb)||
  (skb_cloned(skb) && !skb_clone_writable(skb, 0))) {
  struct sk_buff *new_skb = skb_realloc_headroom(skb, max_headroom);
+ if (max_headroom > dev->needed_headroom)
+ dev->needed_headroom = max_headroom;
  if (!new_skb) {
  ip_rt_put(rt);
  txq->tx_dropped++;