selftests: ublk: make sure _add_ublk_dev can return in sub-shell

Detach ublk daemon from the starting process completely by double-fork and
clearing its process group, so that `_add_ublk_dev` can return from sub-shell.

Then it is more friendly for writing shell test script for adding/recovering
ublk device.

Prepare for running ublk test in parallel.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-5-ming.lei@redhat.com


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