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.h b/drivers/net/ethernet/intel/igb/igb.h
index bf9bf90..be35edc 100644
--- a/drivers/net/ethernet/intel/igb/igb.h
+++ b/drivers/net/ethernet/intel/igb/igb.h
@@ -563,6 +563,7 @@ struct igb_adapter {
 	struct cyclecounter cc;
 	struct timecounter tc;
 	u32 tx_hwtstamp_timeouts;
+	u32 tx_hwtstamp_skipped;
 	u32 rx_hwtstamp_cleared;
 	bool pps_sys_wrap_on;