igb: Correcting and improving small packet check and padding

Current implementation mess up the tail pointer. This patch sets skb->tail
correctly.
Also, the small packet check and padding is optimized by using unlikely and
calling skb_pad directly.

Signed-off-by: Tushar Dave <tushar.n.dave@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -4467,10 +4467,11 @@ static netdev_tx_t igb_xmit_frame(struct sk_buff *skb,
 	 * The minimum packet size with TCTL.PSP set is 17 so pad the skb
 	 * in order to meet this minimum size requirement.
 	 */
-	if (skb->len < 17) {
-		if (skb_padto(skb, 17))
+	if (unlikely(skb->len < 17)) {
+		if (skb_pad(skb, 17 - skb->len))
 			return NETDEV_TX_OK;
 		skb->len = 17;
+		skb_set_tail_pointer(skb, 17);
 	}
 
 	return igb_xmit_frame_ring(skb, igb_tx_queue_mapping(adapter, skb));