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_ethtool.c b/drivers/net/ethernet/intel/igb/igb_ethtool.c
index 0efb62d..8730f7c 100644
--- a/drivers/net/ethernet/intel/igb/igb_ethtool.c
+++ b/drivers/net/ethernet/intel/igb/igb_ethtool.c
@@ -90,6 +90,7 @@ static const struct igb_stats igb_gstrings_stats[] = {
 	IGB_STAT("os2bmc_tx_by_host", stats.o2bspc),
 	IGB_STAT("os2bmc_rx_by_host", stats.b2ogprc),
 	IGB_STAT("tx_hwtstamp_timeouts", tx_hwtstamp_timeouts),
+	IGB_STAT("tx_hwtstamp_skipped", tx_hwtstamp_skipped),
 	IGB_STAT("rx_hwtstamp_cleared", rx_hwtstamp_cleared),
 };