[S390] topology: fix cpu masks for topology=off case

Fix cpu masks for 'topology=off' case. Folding of the scheduling domains
happen in such a way that everything belongs to the MC domain instead
of the CPU doimain.
This should fix a performance regression introduced with
eafd2b6d "[S390] topology: use default MC domain initializer" and also
makes sure we have the same behavious as if CONFIG_SCHED_MC was not
selected at all.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/topology.c

@@ -53,8 +53,10 @@ static cpumask_t cpu_group_map(struct mask_info *info, unsigned int cpu)
 	cpumask_t mask;
 
 	cpus_clear(mask);
-	if (!topology_enabled || !MACHINE_HAS_TOPOLOGY)
-		return cpu_possible_map;
+	if (!topology_enabled || !MACHINE_HAS_TOPOLOGY) {
+		cpumask_copy(&mask, cpumask_of(cpu));
+		return mask;
+	}
 	while (info) {
 		if (cpu_isset(cpu, info->mask)) {
 			mask = info->mask;