selftests: ublk: fix parsing '-a' argument

The argument of '-a' doesn't follow any value, so fix it by putting it
with '-z' together.

Fixes: bedc9cbc ("selftests: ublk: add ublk zero copy test")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250303124324.3563605-5-ming.lei@redhat.com


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