pktgen: remove ndelay() call

Daniel Turull reported inaccuracies in pktgen when using low packet
rates, because we call ndelay(val) with values bigger than 20000.

Instead of calling ndelay() for delays < 100us, we can instead loop
calling ktime_now() only.
Reported-by: NDaniel Turull <daniel.turull@gmail.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -2145,9 +2145,12 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
 	}
 
 	start_time = ktime_now();
-	if (remaining < 100000)
-		ndelay(remaining);	/* really small just spin */
-	else {
+	if (remaining < 100000) {
+		/* for small delays (<100us), just loop until limit is reached */
+		do {
+			end_time = ktime_now();
+		} while (ktime_lt(end_time, spin_until));
+	} else {
 		/* see do_nanosleep */
 		hrtimer_init_sleeper(&t, current);
 		do {
@@ -2162,8 +2165,8 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
 			hrtimer_cancel(&t.timer);
 		} while (t.task && pkt_dev->running && !signal_pending(current));
 		__set_current_state(TASK_RUNNING);
+		end_time = ktime_now();
 	}
-	end_time = ktime_now();
 
 	pkt_dev->idle_acc += ktime_to_ns(ktime_sub(end_time, start_time));
 	pkt_dev->next_tx = ktime_add_ns(spin_until, pkt_dev->delay);