lightnvm: pblk: return NVM_ error on failed submission

Return a meaningful error when the sanity vector I/O check fails.

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