xfs: document the invalidate_bdev call in invalidate_bdev

Copy and paste the commit message from Darrick into a comment to explain
the seemingly odd invalidate_bdev in xfs_shutdown_devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Message-Id: <20230809220545.1308228-8-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>