s390/smp: 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: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20230313182918.1312597-19-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/s390/kernel/smp.c

@@ -1227,11 +1227,17 @@ static DEVICE_ATTR_WO(rescan);
 
 static int __init s390_smp_init(void)
 {
+	struct device *dev_root;
 	int cpu, rc = 0;
 
-	rc = device_create_file(cpu_subsys.dev_root, &dev_attr_rescan);
-	if (rc)
-		return rc;
+	dev_root = bus_get_dev_root(&cpu_subsys);
+	if (dev_root) {
+		rc = device_create_file(dev_root, &dev_attr_rescan);
+		put_device(dev_root);
+		if (rc)
+			return rc;
+	}
+
 	for_each_present_cpu(cpu) {
 		rc = smp_add_present_cpu(cpu);
 		if (rc)