ext4: use fs_holder_ops for the log device

Use the generic fs_holder_ops to shut down the file system when the
log device goes away instead of duplicating the logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230802154131.2221419-11-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>