ext4: close the external journal device in ->kill_sb

blkdev_put must not be called under sb->s_umount to avoid a lock order
reversal with disk->open_mutex.  Move closing the external journal device
into ->kill_sb to archive that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230809220545.1308228-9-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>