iommu/vt-d: Clean up iommu_no_mapping

Invert the return value to avoid double negatives, use a bool
instead of int as the return value, and reduce some indentation
after early returns.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>