iommufd/selftest: Check the bus type during probe

This relied on the probe function only being invoked by the bus type mock
was registered on. The removal of the bus ops broke this assumption and
the probe could be called on non-mock bus types like PCI.

Check the bus type directly in probe.

Fixes: 17de3f5f ("iommu: Retire bus ops")
Link: https://lore.kernel.org/r/0-v1-82d59f7eab8c+40c-iommufd_mock_bus_jgg@nvidia.com


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