drivers/block/null_blk/main: Fix a double free in null_init.

In null_init, null_add_dev(dev) is called.
In null_add_dev, it calls null_free_zoned_dev(dev) to free dev->zones
via kvfree(dev->zones) in out_cleanup_zone branch and returns err.
Then null_init accept the err code and then calls null_free_dev(dev).

But in null_free_dev(dev), dev->zones is freed again by
null_free_zoned_dev().

My patch set dev->zones to NULL in null_free_zoned_dev() after
kvfree(dev->zones) is called, to avoid the double free.

Fixes: 2984c86 ("nullb: factor disk parameters")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Link: https://lore.kernel.org/r/20210426143229.7374-1-lyl2019@mail.ustc.edu.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/zoned.c

@@ -180,6 +180,7 @@ int null_register_zoned_dev(struct nullb *nullb)
 void null_free_zoned_dev(struct nullb_device *dev)
 {
 	kvfree(dev->zones);
+	dev->zones = NULL;
 }
 
 int null_report_zones(struct gendisk *disk, sector_t sector,