ixgbe: add statistic indicating number of skipped Tx timestamps

ixgbe: add statistic indicating number of skipped Tx timestamps

The ixgbe 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: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Orabug: 26785078
(cherry picked from commit 4cc74c01ef8bb59fae98aeda359e8bcf6148943a)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Ethan Zhao <ethan.zhao@oracle.com>