cpu/hotplug: move to use bus_get_dev_root()

Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Phil Auld <pauld@redhat.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20230313182918.1312597-7-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/cpu.c

@@ -2569,22 +2569,33 @@ static const struct attribute_group cpuhp_smt_attr_group = {
 
 static int __init cpu_smt_sysfs_init(void)
 {
-	return sysfs_create_group(&cpu_subsys.dev_root->kobj,
-				  &cpuhp_smt_attr_group);
+	struct device *dev_root;
+	int ret = -ENODEV;
+
+	dev_root = bus_get_dev_root(&cpu_subsys);
+	if (dev_root) {
+		ret = sysfs_create_group(&dev_root->kobj, &cpuhp_smt_attr_group);
+		put_device(dev_root);
+	}
+	return ret;
 }
 
 static int __init cpuhp_sysfs_init(void)
 {
+	struct device *dev_root;
 	int cpu, ret;
 
 	ret = cpu_smt_sysfs_init();
 	if (ret)
 		return ret;
 
-	ret = sysfs_create_group(&cpu_subsys.dev_root->kobj,
-				 &cpuhp_cpu_root_attr_group);
+	dev_root = bus_get_dev_root(&cpu_subsys);
+	if (dev_root) {
+		ret = sysfs_create_group(&dev_root->kobj, &cpuhp_cpu_root_attr_group);
+		put_device(dev_root);
 		if (ret)
 			return ret;
+	}
 
 	for_each_possible_cpu(cpu) {
 		struct device *dev = get_cpu_device(cpu);