lightnvm: pblk: free write buffer on init failure

Refactor the way we free the write buffer to ensure that all entries get
freed in case of an error on the init sequence.

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