selftests: ublk: enable zero copy for null target

Enable zero copy for null target so that we can evaluate performance
from zero copy or not.

Also this should be the simplest ublk zero copy implementation, which
can be served as zc example.

Add test for covering 'add -t null -z'.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-7-ming.lei@redhat.com


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