nbd: fix a block_device refcount leak in nbd_release

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

Fixes: 08ba91e ("nbd: Add the nbd NBD_DISCONNECT_ON_CLOSE config flag.")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

drivers/block/nbd.c

@@ -1518,6 +1518,7 @@ 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);