x86/umwait: 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: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
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/20230313182918.1312597-10-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kernel/cpu/umwait.c

@@ -232,7 +232,11 @@ static int __init umwait_init(void)
 	 * Add umwait control interface. Ignore failure, so at least the
 	 * default values are set up in case the machine manages to boot.
 	 */
-	dev = cpu_subsys.dev_root;
-	return sysfs_create_group(&dev->kobj, &umwait_attr_group);
+	dev = bus_get_dev_root(&cpu_subsys);
+	if (dev) {
+		ret = sysfs_create_group(&dev->kobj, &umwait_attr_group);
+		put_device(dev);
+	}
+	return ret;
 }
 device_initcall(umwait_init);