IB/srp: 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: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>