iommufd: Register iommufd mock devices with fwspec

iommufd: Register iommufd mock devices with fwspec

Since the bus ops were retired the iommu subsystem changed to using fwspec
to match the iommu driver to the iommu device. If a device has a NULL
fwspec then it is matched to the first iommu driver with a NULL fwspec,
effectively disabling support for systems with more than one non-fwspec
iommu driver.

Thus, if the iommufd selfest are run in an x86 system that registers a
non-fwspec iommu driver they fail to bind their mock devices to the mock
iommu driver.

Fix this by allocating a software fwnode for mock iommu driver's
iommu_device, and set it to the device which mock iommu driver created.

This is done by adding a new helper iommu_mock_device_add() which abuses
the internals of the fwspec system to establish a fwspec before the device
is added and is careful not to leak it. A matching dummy fwspec is
automatically added to the mock iommu driver.

Test by "make -C toosl/testing/selftests TARGETS=iommu run_tests":
PASSED: 229 / 229 tests passed.

In addition, this issue is also can be found on amd platform, and
also tested on a amd machine.

Link: https://patch.msgid.link/r/20250925054730.3877-1-kanie@linux.alibaba.com
Fixes: 17de3f5fdd35 ("iommu: Retire bus ops")
Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>