scftorture: Adapt memory-ordering test to UP operation

On uniprocessor systems, smp_call_function() does nothing.  This commit
therefore avoids complaining about the lack of handler accesses in the
single-CPU case where there is no handler.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/scftorture.c

@@ -363,7 +363,8 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 			scfcp->scfc_out = true;
 	}
 	if (scfcp && scfsp->scfs_wait) {
-		if (WARN_ON_ONCE(!scfcp->scfc_out))
+		if (WARN_ON_ONCE((num_online_cpus() > 1 || scfsp->scfs_prim == SCF_PRIM_SINGLE) &&
+				 !scfcp->scfc_out))
 			atomic_inc(&n_mb_out_errs); // Leak rather than trash!
 		else
 			kfree(scfcp);