RDMA/rw: drop pci_p2pdma_[un]map_sg()

dma_map_sg() now supports the use of P2PDMA pages so pci_p2pdma_map_sg()
is no longer necessary and may be dropped. This means the
rdma_rw_[un]map_sg() helpers are no longer necessary. Remove it all.

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