lightnvm: wrong offset in bad blk lun calculation

dev->nr_luns reports the total number of luns available in a device
while dev->luns_per_chnl is the number of luns per channel.

When multiple channels are available, the offset is calculated from a
channel and lun id into a linear array. As it multiplies with
the total number of luns, we go out of bound when channel id > 0 and
causes the kernel to panic when we read a protected kernel memory area.

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