rcutorture: Bounds-check rcutorture.shuffle_interval

Specifying a negative rcutorture.shuffle_interval value will cause a
negative value to be used as a sleep time.  This commit therefore
refuses to start shuffling unless the rcutorture.shuffle_interval
value is greater than zero.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -1821,7 +1821,7 @@ rcu_torture_init(void)
 		if (firsterr)
 			goto unwind;
 	}
-	if (test_no_idle_hz) {
+	if (test_no_idle_hz && shuffle_interval > 0) {
 		firsterr = torture_shuffle_init(shuffle_interval * HZ);
 		if (firsterr)
 			goto unwind;