lightnvm: fix bad back free on error path

Free memory correctly when an allocation fails on a loop and we free
backwards previously successful allocations.

Signed-off-by: Javier González <javier@cnexlabs.com>
Reviewed-by: Matias Bjørling <matias@cnexlabs.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>