ixgbevf: fix handling of tx checksumming

This patch resolves an issue introduced by:
commit 7ad1a093
ixgbevf: make the first tx_buffer a repository for most of the skb info

Incorrect check for the result of ixgbevf_tso() can lead to calling
ixgbevf_tx_csum() which can spawn 2 context descriptors and result in
performance degradation and/or corrupted packets.
Signed-off-by: NEmil Tantilov <emil.s.tantilov@intel.com>
Tested-by: NPhil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c

@@ -3155,7 +3155,7 @@ static int ixgbevf_xmit_frame(struct sk_buff *skb, struct net_device *netdev)
 	tso = ixgbevf_tso(tx_ring, first, &hdr_len);
 	if (tso < 0)
 		goto out_drop;
-	else
+	else if (!tso)
 		ixgbevf_tx_csum(tx_ring, first);
 
 	ixgbevf_tx_map(tx_ring, first, hdr_len);