fix free_request_tracking_ptr set dev->sd_orig_request_fn as null

[hongyuntw]

If we are using submit_bio kernel version, we also need to reset sd_orig_request_fn to null when dattobd_free_request_tracking_ptr is called. Since __tracer_destroy_tracing determined to do destroy traced or not by checking if sd_orig_request_fn is a nullptr, not doing this might cause a kernel panic due to double destroy cases.