igb: Free any held skb that should have been timestamped on remove

To prevent a race condition where an skb has been saved to return the Tx
timestamp later and the driver is removed, add a check to determine if we
have an skb stored and, if so, free it.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-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>