lib/Kconfig.debug: add list_sort debugging switch

While hunting a non-existing bug in 'list_sort()', I've improved the
'list_sort_test()' function which tests the 'list_sort()' library call.
Although at the end I found a bug in my code, but not in 'list_sort()', I
think my clean-ups and improvements are worth merging because they make
the test function better.

This patch:

Make the self-tests selectable via Kconfig rather than by manual enabling
of DEBUG_LIST_SORT.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: Don Mullis <don.mullis@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.debug

@@ -748,6 +748,15 @@ config DEBUG_LIST
 
 	  If unsure, say N.
 
+config TEST_LIST_SORT
+	bool "Linked list sorting test"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on 'list_sort()' function test. This test is
+	  executed only once during system boot, so affects only boot time.
+
+	  If unsure, say N.
+
 config DEBUG_SG
 	bool "Debug SG table operations"
 	depends on DEBUG_KERNEL

lib/list_sort.c

@@ -141,7 +141,7 @@ void list_sort(void *priv, struct list_head *head,
 }
 EXPORT_SYMBOL(list_sort);
 
-#ifdef DEBUG_LIST_SORT
+#ifdef CONFIG_TEST_LIST_SORT
 struct debug_el {
 	struct list_head l_h;
 	int value;
@@ -214,4 +214,4 @@ static int __init list_sort_test(void)
 	return 0;
 }
 module_init(list_sort_test);
-#endif
+#endif /* CONFIG_TEST_LIST_SORT */