vfio: Do not allow !ops->dma_unmap in vfio_pin/unpin_pages()

A driver that doesn't implement ops->dma_unmap shouldn't be allowed to do
vfio_pin/unpin_pages(), though it can use vfio_dma_rw() to access an iova
range. Deny !ops->dma_unmap cases in vfio_pin/unpin_pages().

Link: https://lore.kernel.org/r/85d622729d8f2334b35d42f1c568df1ededb9171.1690523699.git.nicolinc@nvidia.com
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>