rcutorture: Preempt RCU-preempt readers more vigorously

This commit attempts to make a very rare rcutorture failure happen
more often by increasing the fraction of RCU-preempt read-side critical
sections that are preempted.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -316,7 +316,7 @@ static void rcu_read_delay(struct torture_random_state *rrsp)
 	if (!(torture_random(rrsp) % (nrealreaders * 2 * shortdelay_us)))
 		udelay(shortdelay_us);
 	if (!preempt_count() &&
-	    !(torture_random(rrsp) % (nrealreaders * 20000)))
+	    !(torture_random(rrsp) % (nrealreaders * 500)))
 		torture_preempt_schedule();  /* QS only if preemptible. */
 }