lightnvm: fail fast on passthrough commands

Make LightNVM passhtrough commands fail fast. User space will then take
care of re-submitting.

Fixes: 84d4add7 ('lightnvm: add ioctls for vector I/Os')
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>