ACPI: LPIT: 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: Len Brown <lenb@kernel.org>
Cc: linux-acpi@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20230322083646.2937580-1-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/acpi/acpi_lpit.c

@@ -98,6 +98,12 @@ EXPORT_SYMBOL_GPL(lpit_read_residency_count_address);
 static void lpit_update_residency(struct lpit_residency_info *info,
 				 struct acpi_lpit_native *lpit_native)
 {
+	struct device *dev_root = bus_get_dev_root(&cpu_subsys);
+
+	/* Silently fail, if cpuidle attribute group is not present */
+	if (!dev_root)
+		return;
+
 	info->frequency = lpit_native->counter_frequency ?
 				lpit_native->counter_frequency : tsc_khz * 1000;
 	if (!info->frequency)
@@ -108,18 +114,18 @@ static void lpit_update_residency(struct lpit_residency_info *info,
 		info->iomem_addr = ioremap(info->gaddr.address,
 						   info->gaddr.bit_width / 8);
 		if (!info->iomem_addr)
-			return;
+			goto exit;
 
-		/* Silently fail, if cpuidle attribute group is not present */
-		sysfs_add_file_to_group(&cpu_subsys.dev_root->kobj,
+		sysfs_add_file_to_group(&dev_root->kobj,
 					&dev_attr_low_power_idle_system_residency_us.attr,
 					"cpuidle");
 	} else if (info->gaddr.space_id == ACPI_ADR_SPACE_FIXED_HARDWARE) {
-		/* Silently fail, if cpuidle attribute group is not present */
-		sysfs_add_file_to_group(&cpu_subsys.dev_root->kobj,
+		sysfs_add_file_to_group(&dev_root->kobj,
 					&dev_attr_low_power_idle_cpu_residency_us.attr,
 					"cpuidle");
 	}
+exit:
+	put_device(dev_root);
 }
 
 static void lpit_process(u64 begin, u64 end)