lightnvm: use constant name instead of value

There is a constant to refer to free blocks. Use it when marking bad
blocks instead of using a constant value

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