of: unittest: Fix memory leak in unittest_data_add()

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 <robh@kernel.org>
Co-developed-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Link: https://patch.msgid.link/20251231114915.234638-1-zilin@seu.edu.cn
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>