cleanup: Provide retain_ptr()

In cases where an allocation is consumed by another function, the
allocation needs to be retained on success or freed on failure. The code
pattern is usually:

	struct foo *f = kzalloc(sizeof(*f), GFP_KERNEL);
	struct bar *b;

	,,,
	// Initialize f
	...
	if (ret)
		goto free;
        ...
	bar = bar_create(f);
	if (!bar) {
		ret = -ENOMEM;
	   	goto free;
	}
	...
	return 0;
free:
	kfree(f);
	return ret;

This prevents using __free(kfree) on @f because there is no canonical way
to tell the cleanup code that the allocation should not be freed.

Abusing no_free_ptr() by force ignoring the return value is not really a
sensible option either.

Provide an explicit macro retain_ptr(), which NULLs the cleanup
pointer. That makes it easy to analyze and reason about.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/all/20250313130321.442025758@linutronix.de

include/linux/cleanup.h

@@ -216,6 +216,23 @@ const volatile void * __must_check_fn(const volatile void *val)
 
 #define return_ptr(p)	return no_free_ptr(p)
 
+/*
+ * Only for situations where an allocation is handed in to another function
+ * and consumed by that function on success.
+ *
+ *	struct foo *f __free(kfree) = kzalloc(sizeof(*f), GFP_KERNEL);
+ *
+ *	setup(f);
+ *	if (some_condition)
+ *		return -EINVAL;
+ *	....
+ *	ret = bar(f);
+ *	if (!ret)
+ *		retain_ptr(f);
+ *	return ret;
+ */
+#define retain_ptr(p)				\
+	__get_and_null(p, NULL)
 
 /*
  * DEFINE_CLASS(name, type, exit, init, init_args...):