selftests: ublk: load/unload ublk_drv when preparing & cleaning up tests

Load ublk_drv module in _prep_test(), and unload it in _cleanup_test(),
so that test can always be done in consistent state.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250303124324.3563605-9-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>