igb: Add support for SW timestamping

Enable SW timestamping for situations where the user may prefer it over HW
timestamping or there may not be HW timestamping.

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>
diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index a9cb84a..9dd3da5 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -4610,6 +4610,8 @@
 	first->bytecount = skb->len;
 	first->gso_segs = 1;
 
+	skb_tx_timestamp(skb);
+
 	if (unlikely((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
 		     !(adapter->ptp_tx_skb))) {
 		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;