of: unittest: Fix memory leak in unittest_data_add()

In unittest_data_add(), if of_resolve_phandles() fails, the allocated
unittest_data is not freed, leading to a memory leak.

Fix this by using scope-based cleanup helper __free(kfree) for automatic
resource cleanup. This ensures unittest_data is automatically freed when
it goes out of scope in error paths.

For the success path, use retain_and_null_ptr() to transfer ownership
of the memory to the device tree and prevent double freeing.

Fixes: 2eb46da2a760 ("of/selftest: Use the resolver to fixup phandles")
Suggested-by: Rob Herring <[e-mail]>
Co-developed-by: Jianhao Xu <[e-mail]>
Signed-off-by: Jianhao Xu <[e-mail]>
Signed-off-by: Zilin Guan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

drivers/of/unittest.c

···
  */
 static int __init unittest_data_add(void)
 {
-	void *unittest_data;
 	void *unittest_data_align;
 	struct device_node *unittest_data_node = NULL, *np;
 	/*
···
 	}
 
 	/* creating copy */
-	unittest_data = kmalloc(size + FDT_ALIGN_SIZE, GFP_KERNEL);
+	void *unittest_data __free(kfree) = kmalloc(size + FDT_ALIGN_SIZE, GFP_KERNEL);
 	if (!unittest_data)
 		return -ENOMEM;
 
···
 	ret = of_fdt_unflatten_tree(unittest_data_align, NULL, &unittest_data_node);
 	if (!ret) {
 		pr_warn("%s: unflatten testcases tree failed\n", __func__);
-		kfree(unittest_data);
 		return -ENODATA;
 	}
 	if (!unittest_data_node) {
 		pr_warn("%s: testcases tree is empty\n", __func__);
-		kfree(unittest_data);
 		return -ENODATA;
 	}
 
···
 	/* attach the sub-tree to live tree */
 	if (!of_root) {
 		pr_warn("%s: no live tree to attach sub-tree\n", __func__);
-		kfree(unittest_data);
 		rc = -ENODEV;
 		goto unlock;
 	}
···
 
 	EXPECT_END(KERN_INFO,
 		   "Duplicate name in testcase-data, renamed to \"duplicate-name#1\"");
+
+	retain_and_null_ptr(unittest_data);
 
 unlock:
 	of_overlay_mutex_unlock();