dm vdo io-submitter: rename to vdo_submit_vio and submit_data_vio

Rename process_vio_io() to vdo_submit_vio(), and process_data_vio_io() to
submit_data_vio().

Reviewed-by: Susan LeGendre-McGhee <slegendr@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>