CVE-2026-23137: of: unittest: Fix memory leak in unittest_data_add()

In the Linux kernel, the following vulnerability has been resolved: 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.