refperf: Allow decimal nanoseconds

The CONFIG_PREEMPT=n rcu_read_lock()/rcu_read_unlock() pair's overhead,
even including loop overhead, is far less than one nanosecond.
Since logscale plots are not all that happy with zero values, provide
picoseconds as decimals.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -375,7 +375,7 @@ static int main_func(void *arg)
 		if (torture_must_stop())
 			goto end;
 
-		reader_tasks[exp].result_avg = process_durations(exp) / ((exp + 1) * loops);
+		reader_tasks[exp].result_avg = 1000 * process_durations(exp) / ((exp + 1) * loops);
 	}
 
 	// Print the average of all experiments
@@ -386,7 +386,7 @@ static int main_func(void *arg)
 	strcat(buf, "Threads\tTime(ns)\n");
 
 	for (exp = 0; exp < nreaders; exp++) {
-		sprintf(buf1, "%d\t%llu\n", exp + 1, reader_tasks[exp].result_avg);
+		sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, reader_tasks[exp].result_avg / 1000, (int)(reader_tasks[exp].result_avg % 1000));
 		strcat(buf, buf1);
 	}