ip_gre: check packet length and mtu correctly in erspan tx

Similarly to early patch for erspan_xmit(), the ARPHDR_ETHER device
is the length of the whole ether packet.  So skb->len should subtract
the dev->hard_header_len.

Fixes: 1a66a836 ("gre: add collect_md mode to ERSPAN tunnel")
Fixes: 84e54fe0 ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: David Laight <David.Laight@aculab.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -579,8 +579,8 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
 	if (gre_handle_offloads(skb, false))
 		goto err_free_rt;
 
-	if (skb->len > dev->mtu) {
-		pskb_trim(skb, dev->mtu);
+	if (skb->len > dev->mtu + dev->hard_header_len) {
+		pskb_trim(skb, dev->mtu + dev->hard_header_len);
 		truncate = true;
 	}
 
@@ -731,8 +731,8 @@ static netdev_tx_t erspan_xmit(struct sk_buff *skb,
 	if (skb_cow_head(skb, dev->needed_headroom))
 		goto free_skb;
 
-	if (skb->len - dev->hard_header_len > dev->mtu) {
-		pskb_trim(skb, dev->mtu);
+	if (skb->len > dev->mtu + dev->hard_header_len) {
+		pskb_trim(skb, dev->mtu + dev->hard_header_len);
 		truncate = true;
 	}