vfio: Add external user check extension interface

This lets us check extensions, particularly VFIO_DMA_CC_IOMMU using
the external user interface, allowing KVM to probe IOMMU coherency.

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

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

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