igb: add statistic indicating number of skipped Tx timestamps

The igb driver can only handle one Tx timestamp request at a time.
This means it is possible for an application timestamp request to be
ignored.

There is no easy way for an administrator to determine if this occurred.
Add a new statistic which tracks this, tx_hwtstamp_skipped.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index fefa461..06b81a60 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -5388,6 +5388,8 @@ netdev_tx_t igb_xmit_frame_ring(struct sk_buff *skb,
 			adapter->ptp_tx_start = jiffies;
 			if (adapter->hw.mac.type == e1000_82576)
 				schedule_work(&adapter->ptp_tx_work);
+		} else {
+			adapter->tx_hwtstamp_skipped++;
 		}
 	}