lightnvm: fix incorrect nr_free_blocks stat

When initing bad block list in gennvm_block_bb, once we move bad block
from free_list to bb_list, we should maintain both stat info
nr_free_blocks and nr_bad_blocks. So this patch fixes to add missing
operation related to nr_free_blocks.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>