iommu: Do more input validation in iommu_map_sg()

The IOMMU-API works on page boundarys, unlike the DMA-API
which can work with sub-page buffers. The sg->offset
field does not make sense on the IOMMU level, so force it to
be 0. Do some error-path consolidation while at it.

Signed-off-by: Joerg Roedel <jroedel@suse.de>