lightnvm: pblk: fix double-free on pblk init

Prevent pblk->lines being double freed in case of an error during pblk
initialization.

Fixes: dd2a4343: "lightnvm: pblk: sched. metadata on write thread"
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>