selftests: ublk: move zero copy feature check into _add_ublk_dev()

Move zero copy feature check into _add_ublk_dev() since we will have
more tests which requires to cover zero copy.

Then one check function of _check_add_dev() has to be added for dealing
with cleanup since '_add_ublk_dev()' is run in sub-shell, and we can't
exit from it to terminal shell.

Meantime always return error code from _add_ublk_dev().

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250303124324.3563605-8-ming.lei@redhat.com


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