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>

drivers/lightnvm/gennvm.c

@@ -75,7 +75,7 @@ static int gennvm_block_bb(struct ppa_addr ppa, int nr_blocks, u8 *blks,
 	struct nvm_block *blk;
 	int i;
 
-	lun = &gn->luns[(dev->nr_luns * ppa.g.ch) + ppa.g.lun];
+	lun = &gn->luns[(dev->luns_per_chnl * ppa.g.ch) + ppa.g.lun];
 
 	for (i = 0; i < nr_blocks; i++) {
 		if (blks[i] == 0)