iommu: Remove the ->map_sg indirection

All iommu drivers use the default_iommu_map_sg implementation, and there
is no good reason to ever override it.  Just expose it as iommu_map_sg
directly and remove the indirection, specially in our post-spectre world
where indirect calls are horribly expensive.

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