lightnvm: initialize ppa_addr in dev_to_generic_addr()

The ->reserved bit is not initialized when allocated on stack.
This may lead targets to misinterpret the PPA as cached.

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