vfio/type1: Add extension to test DMA cache coherence of IOMMU

Now that the type1 IOMMU backend can support IOMMU_CACHE, we need to
be able to test whether coherency is currently enforced.  Add an
extension for this.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

diff --cc drivers/vfio/vfio_iommu_type1.c
Simple merge

diff --cc include/uapi/linux/vfio.h
Simple merge