lightnvm: fix bad block initialization

fix reading bad block device information to correctly setup the per line
blk_bitmap during lightnvm initialization

Signed-off-by: Heiner Litz <hlitz@ucsc.edu>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>