nvme-rdma: Switch from dma_device to dev.parent

Prepare for removal of ib_device.dma_device.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Acked-by: Sagi Grimberg <sagi@grimbeg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>