lightnvm: pblk: fail gracefully on line alloc. failure

In the event of a line failing to allocate, fail gracefully and stop the
pipeline to avoid more write failing in the same place.

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