lightnvm: prevent gennvm module unload on use

After the gennvm module has been initialized. It might be attached to
one or several devices. In that case, the module is in use. Make sure
that it can not be unloaded.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>