null_blk: add lightnvm null_blk device to the nullb_list

After register null_blk devices into lightnvm, we forget
to add these devices to the the nullb_list, makes them
invisible to the null_blk driver.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Fixes: a514379b0c77 ("null_blk: oops when initializing without lightnvm")
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -742,10 +742,11 @@ static int null_add_dev(void)
 
 	add_disk(disk);
 
+done:
 	mutex_lock(&lock);
 	list_add_tail(&nullb->list, &nullb_list);
 	mutex_unlock(&lock);
-done:
+
 	return 0;
 
 out_cleanup_lightnvm: