ublk: mention WRITE_ZEROES in comment of ublk_complete_rq()

WRITE_ZEROES won't return bytes returned just like FLUSH and DISCARD,
and we can end it directly. Add missing comment for it in
ublk_complete_rq().

Signed-off-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230207070839.370817-3-ZiyangZhang@linux.alibaba.com


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