iommufd/viommu: Support get_viommu_size and viommu_init ops

To ease the for-driver iommufd APIs, get_viommu_size and viommu_init ops
are introduced to replace the viommu_init op.

Let the new viommu_init pathway coexist with the old viommu_alloc one.

Since the viommu_alloc op and its pathway will be soon deprecated, try to
minimize the code difference between them by adding a tentative jump tag.

Note that this fails a !viommu->ops case from now on with a WARN_ON_ONCE
since a vIOMMU is expected to support an alloc_domain_nested op for now,
or some sort of a viommu op in the foreseeable future. This WARN_ON_ONCE
can be lifted, if some day there is a use case wanting !viommu->ops.

Link: https://patch.msgid.link/r/35c5fa5926be45bda82f5fc87545cd3180ad4c9c.1749882255.git.nicolinc@nvidia.com


Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>