PCI/P2PDMA: Factor out __pci_p2pdma_map_sg()

Factor out the bus-only mapping into its own static function.  No
functional changes. The original pci_p2pdma_map_sg_attrs() will be used to
decide whether this is an appropriate way to map.

Link: https://lore.kernel.org/r/20190730163545.4915-11-logang@deltatee.com
Link: https://lore.kernel.org/r/20190812173048.9186-11-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>