selftests: ublk: don't show `modprobe` failure

ublk_drv may be built-in, so don't show modprobe failure, and we
do check `/dev/ublk-control` for skipping test if ublk_drv isn't
enabled.

Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250320013743.4167489-3-ming.lei@redhat.com


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