s390/crashdump: use list_first_entry_or_null

The combo of list_empty() check and return list_first_entry()
can be replaced with list_first_entry_or_null().
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/crash_dump.c

@@ -71,9 +71,7 @@ struct save_area * __init save_area_alloc(bool is_boot_cpu)
  */
 struct save_area * __init save_area_boot_cpu(void)
 {
-	if (list_empty(&dump_save_areas))
-		return NULL;
-	return list_first_entry(&dump_save_areas, struct save_area, list);
+	return list_first_entry_or_null(&dump_save_areas, struct save_area, list);
 }
 
 /*