lightnvm: pblk: set metadata list for all I/Os

Set a dma area for all I/Os in order to read/write from/to the metadata
stored on the per-sector out-of-bound area.

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