loop: remove most the top-of-file boilerplate comment

Remove the irrelevant changelogs and todo notes and just leave the SPDX
marker and the copyright notice.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220419063303.583106-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>