lightnvm: pblk: use nvm_rq_to_ppa_list()

This patch replaces few remaining usages of rqd->ppa_list[] with
existing nvm_rq_to_ppa_list() helpers. This is needed for theoretical
devices with ws_min/ws_opt equal to 1.

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>