lightnvm: double-clear of dev->lun_map on target init error

The dev->lun_map bits are cleared twice if an target init error occurs.
First in the target clean routine, and then next in the nvm_tgt_create
error function. Make sure that it is only cleared once by extending
nvm_remove_tgt_devi() with a clear bit, such that clearing of bits can
ignored when cleaning up a successful initialized target.

Signed-off-by: Javier González <javier@cnexlabs.com>
Fix style.
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>

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