lightnvm: fix lun offset calculation for mark blk

The gen_mark_blk_bad function marks the wrong block when a block is on
a different channel. Fix the index calculation, so that it updates the
correct block.

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