lib/dma-virt: Add dma_virt_ops

Several RDMA drivers (hfi1, qib and rxe) expect that ib_sge.addr
is a virtual address. Provide DMA mapping operations that are
suitable for these drivers.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>