rcutorture: Preempt rather than block when testing task stalls

Currently, rcu_torture_stall() does a one-jiffy timed wait when
stall_cpu_block is set.  This works, but emits a pointless splat in
CONFIG_PREEMPT=y kernels.  This commit avoids this splat by instead
invoking preempt_schedule() in CONFIG_PREEMPT=y kernels.

This uses an admittedly ugly #ifdef, but abstracted approaches just
looked worse.  A prettier approach would provide a preempt_schedule()
definition with a WARN_ON() for CONFIG_PREEMPT=n kernels, but this seems
quite silly.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -2022,8 +2022,13 @@ static int rcu_torture_stall(void *args)
 			  __func__, raw_smp_processor_id());
 		while (ULONG_CMP_LT((unsigned long)ktime_get_seconds(),
 				    stop_at))
-			if (stall_cpu_block)
+			if (stall_cpu_block) {
+#ifdef CONFIG_PREEMPTION
+				preempt_schedule();
+#else
 				schedule_timeout_uninterruptible(HZ);
+#endif
+			}
 		if (stall_cpu_irqsoff)
 			local_irq_enable();
 		else if (!stall_cpu_block)