lightnvm: pblk: use rqd->end_io for completion

For consistency with the rest of pblk, use rqd->end_io to point to the
function taking care of ending the request on the completion path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>