pktgen: Fix fall-through annotation

Replace "fallthru" with a proper "fall through" annotation.

This fix is part of the ongoing efforts to enabling
-Wimplicit-fallthrough

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -3426,7 +3426,7 @@ xmit_more:
 		net_info_ratelimited("%s xmit error: %d\n",
 				     pkt_dev->odevname, ret);
 		pkt_dev->errors++;
-		/* fallthru */
+		/* fall through */
 	case NETDEV_TX_BUSY:
 		/* Retry it next time */
 		refcount_dec(&(pkt_dev->skb->users));