PCI/P2PDMA: Introduce private pagemap structure

Move the PCI bus offset from the generic dev_pagemap structure to a
specific pci_p2pdma_pagemap structure.

This structure will grow in subsequent patches.

Link: https://lore.kernel.org/r/20190730163545.4915-2-logang@deltatee.com
Link: https://lore.kernel.org/r/20190812173048.9186-2-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>