x86/microcode: 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: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20230313182918.1312597-9-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kernel/cpu/microcode/core.c

@@ -632,6 +632,7 @@ static const struct attribute_group cpu_root_microcode_group = {
 
 static int __init microcode_init(void)
 {
+	struct device *dev_root;
 	struct cpuinfo_x86 *c = &boot_cpu_data;
 	int error;
 
@@ -652,11 +653,15 @@ static int __init microcode_init(void)
 	if (IS_ERR(microcode_pdev))
 		return PTR_ERR(microcode_pdev);
 
-	error = sysfs_create_group(&cpu_subsys.dev_root->kobj, &cpu_root_microcode_group);
+	dev_root = bus_get_dev_root(&cpu_subsys);
+	if (dev_root) {
+		error = sysfs_create_group(&dev_root->kobj, &cpu_root_microcode_group);
+		put_device(dev_root);
 		if (error) {
 			pr_err("Error creating microcode group!\n");
 			goto out_pdev;
 		}
+	}
 
 	/* Do per-CPU setup */
 	schedule_on_each_cpu(setup_online_cpu);