kselftest: factor out list manipulation to a helper

Kees suggest to factor out the list append code to a macro,
since following commits need it, which leads to code duplication.

Suggested-by: Kees Cook <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/kselftest_harness.h

···
 	} \
 } while (0); OPTIONAL_HANDLER(_assert)
 
+/* List helpers */
+#define __LIST_APPEND(head, item) \
+{ \
+	/* Circular linked list where only prev is circular. */ \
+	if (head == NULL) { \
+		head = item; \
+		item->next = NULL; \
+		item->prev = item; \
+		return;	\
+	} \
+	if (__constructor_order == _CONSTRUCTOR_ORDER_FORWARD) { \
+		item->next = NULL; \
+		item->prev = head->prev; \
+		item->prev->next = item; \
+		head->prev = item; \
+	} else { \
+		item->next = head; \
+		item->next->prev = item; \
+		item->prev = item; \
+		head = item; \
+	} \
+}
+
 /* Contains all the information for test execution and status checking. */
 struct __test_metadata {
 	const char *name;
···
 static inline void __register_test(struct __test_metadata *t)
 {
 	__test_count++;
-	/* Circular linked list where only prev is circular. */
-	if (__test_list == NULL) {
-		__test_list = t;
-		t->next = NULL;
-		t->prev = t;
-		return;
-	}
-	if (__constructor_order == _CONSTRUCTOR_ORDER_FORWARD) {
-		t->next = NULL;
-		t->prev = __test_list->prev;
-		t->prev->next = t;
-		__test_list->prev = t;
-	} else {
-		t->next = __test_list;
-		t->next->prev = t;
-		t->prev = t;
-		__test_list = t;
-	}
+	__LIST_APPEND(__test_list, t);
 }
 
 static inline int __bail(int for_realz, bool no_print, __u8 step)