e1000e: add statistic indicating number of skipped Tx timestamps

The e1000e 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>

drivers/net/ethernet/intel/e1000e/e1000.h

@@ -268,6 +268,7 @@ struct e1000_adapter {
 	u32 tx_fifo_size;
 	u32 tx_dma_failed;
 	u32 tx_hwtstamp_timeouts;
+	u32 tx_hwtstamp_skipped;
 
 	/* Rx */
 	bool (*clean_rx)(struct e1000_ring *ring, int *work_done,