iommu: Explicitly skip bus address marked segments in __iommu_map_sg()

In order to support PCI P2PDMA mappings with dma-iommu, explicitly skip
any segments marked with sg_dma_mark_bus_address() in __iommu_map_sg().

These segments should not be mapped into the IOVA and will be handled
separately in as subsequent patch for dma-iommu.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>