iommufd/selftest: Return the real idev id from selftest mock_domain

Now that we actually call iommufd_device_bind() we can return the
idev_id from that function to userspace for use in other APIs.

Link: https://lore.kernel.org/r/16-v4-9cd79ad52ee8+13f5-iommufd_alloc_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>