vfio: Remove #ifdefs around CONFIG_VFIO_NOIOMMU

This can all be accomplished using typical IS_ENABLED techniques, drop it
all.

Also rename the variable to vfio_noiommu so this can be made global in
following patches.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v3-297af71838d2+b9-vfio_container_split_jgg@nvidia.com


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