lightnvm: free properly on target creation error

Fix a memory leak when target creation fails. More specifically, free
the entire device structure given to the target (tgt_dev).

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