lightnvm: do not protect block 0

Device blocks should be marked by the device and considered as bad
blocks by the media manager. Thus, do not make assumptions on which
blocks are going to be used by the device. In doing so we might lose
valid blocks from the free list.

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