s390/topology: clear thread/group maps for offline cpus

The current code doesn't clear the thread/group maps for offline
CPUs. This may cause kernel crashes like the one bewlow in common
code that assumes if a CPU has sibblings it is online.

Unable to handle kernel pointer dereference in virtual kernel address space

Call Trace:
 [<000000013a4b8c3c>] blk_mq_map_swqueue+0x10c/0x388
([<000000013a4b8bcc>] blk_mq_map_swqueue+0x9c/0x388)
 [<000000013a4b9300>] blk_mq_init_allocated_queue+0x448/0x478
 [<000000013a4b9416>] blk_mq_init_queue+0x4e/0x90
 [<000003ff8019d3e6>] loop_add+0x106/0x278 [loop]
 [<000003ff801b8148>] loop_init+0x148/0x1000 [loop]
 [<0000000139de4924>] do_one_initcall+0x3c/0x1e0
 [<0000000139ef449a>] do_init_module+0x6a/0x2a0
 [<0000000139ef61bc>] __do_sys_finit_module+0xa4/0xc0
 [<0000000139de9e6e>] do_syscall+0x7e/0xd0
 [<000000013a8e0aec>] __do_syscall+0xbc/0x110
 [<000000013a8ee2e8>] system_call+0x78/0xa0

Fixes: 52aeda7a ("s390/topology: remove offline CPUs from CPU topology masks")
Cc: <stable@kernel.org> # 5.7+
Reported-by: Marius Hillenbrand <mhillen@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/topology.c

@@ -66,7 +66,10 @@ static void cpu_group_map(cpumask_t *dst, struct mask_info *info, unsigned int c
 {
 	static cpumask_t mask;
 
-	cpumask_copy(&mask, cpumask_of(cpu));
+	cpumask_clear(&mask);
+	if (!cpu_online(cpu))
+		goto out;
+	cpumask_set_cpu(cpu, &mask);
 	switch (topology_mode) {
 	case TOPOLOGY_MODE_HW:
 		while (info) {
@@ -83,10 +86,10 @@ static void cpu_group_map(cpumask_t *dst, struct mask_info *info, unsigned int c
 	default:
 		fallthrough;
 	case TOPOLOGY_MODE_SINGLE:
-		cpumask_copy(&mask, cpumask_of(cpu));
 		break;
 	}
 	cpumask_and(&mask, &mask, cpu_online_mask);
+out:
 	cpumask_copy(dst, &mask);
 }
 
@@ -95,7 +98,10 @@ static void cpu_thread_map(cpumask_t *dst, unsigned int cpu)
 	static cpumask_t mask;
 	int i;
 
-	cpumask_copy(&mask, cpumask_of(cpu));
+	cpumask_clear(&mask);
+	if (!cpu_online(cpu))
+		goto out;
+	cpumask_set_cpu(cpu, &mask);
 	if (topology_mode != TOPOLOGY_MODE_HW)
 		goto out;
 	cpu -= cpu % (smp_cpu_mtid + 1);