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: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NJavier González <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <matias@cnexlabs.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>