nbd: fix a block_device refcount leak in nbd_release

stable inclusion
from linux-4.19.158
commit d9f4534a9a286877ae29e15b5f9ba8ecb7370924

--------------------------------

[ Upstream commit 2bd645b2 ]

bdget_disk needs to be paired with bdput to not leak a reference
on the block device inode.

Fixes: 08ba91ee ("nbd: Add the nbd NBD_DISCONNECT_ON_CLOSE config flag.")
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

drivers/block/nbd.c

@@ -1465,6 +1465,7 @@ static void nbd_release(struct gendisk *disk, fmode_t mode)
 	if (test_bit(NBD_RT_DISCONNECT_ON_CLOSE, &nbd->config->runtime_flags) &&
 			bdev->bd_openers == 0)
 		nbd_disconnect_and_put(nbd);
+	bdput(bdev);
 
 	nbd_config_put(nbd);
 	nbd_put(nbd);