vxlan: Improve vxlan headroom calculation.

Rather than having static headroom calculation, adjust headroom
according to target device.
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1130,6 +1130,7 @@ int vxlan_xmit_skb(struct net *net, struct vxlan_sock *vs,
 {
 	struct vxlanhdr *vxh;
 	struct udphdr *uh;
+	int min_headroom;
 	int err;
 
 	if (!skb->encapsulation) {
@@ -1137,6 +1138,14 @@ int vxlan_xmit_skb(struct net *net, struct vxlan_sock *vs,
 		skb->encapsulation = 1;
 	}
 
+	min_headroom = LL_RESERVED_SPACE(rt->dst.dev) + rt->dst.header_len
+			+ VXLAN_HLEN + sizeof(struct iphdr);
+
+	/* Need space for new headers (invalidates iph ptr) */
+	err = skb_cow_head(skb, min_headroom);
+	if (unlikely(err))
+		return err;
+
 	vxh = (struct vxlanhdr *) __skb_push(skb, sizeof(*vxh));
 	vxh->vx_flags = htonl(VXLAN_FLAGS);
 	vxh->vx_vni = vni;
@@ -1220,10 +1229,6 @@ static void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 		goto drop;
 	}
 
-	/* Need space for new headers (invalidates iph ptr) */
-	if (skb_cow_head(skb, VXLAN_HEADROOM))
-		goto drop;
-
 	old_iph = ip_hdr(skb);
 
 	ttl = vxlan->ttl;