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 <[e-mail]>

kernel/rcu/rcutorture.c

···
 	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. */
 }