scftorture: Add cond_resched() to test loop

Although the test loop does randomly delay, which would provide quiescent
states and so forth, it is possible for there to be a series of long
smp_call_function*() handler runtimes with no delays, which results in
softlockup and RCU CPU stall warning messages.  This commit therefore
inserts a cond_resched() into the main test loop.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/scftorture.c

@@ -420,6 +420,7 @@ static int scftorture_invoker(void *arg)
 			set_cpus_allowed_ptr(current, cpumask_of(cpu));
 			was_offline = false;
 		}
+		cond_resched();
 	} while (!torture_must_stop());
 
 	VERBOSE_SCFTORTOUT("scftorture_invoker %d ended", scfp->cpu);