lightnvm: propagate right error code to target

If nvme_alloc_request fails, propagate the right error, instead of
assuming ENOMEM.

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