vfio: Permit VFIO to work with pinned importers

vfio: Permit VFIO to work with pinned importers

Till now VFIO has rejected pinned importers, largely to avoid being used
with the RDMA pinned importer that cannot handle a move_notify() to revoke
access.

Using dma_buf_attach_revocable() it can tell the difference between pinned
importers that support the flow described in dma_buf_invalidate_mappings()
and those that don't.

Thus permit compatible pinned importers.

This is one of two items IOMMUFD requires to remove its private interface
to VFIO's dma-buf.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Alex Williamson <alex@shazbot.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20260131-dmabuf-revoke-v7-7-463d956bd527@nvidia.com