loop: add a SPDX header

The copyright statement says:

"Redistribution of this file is permitted under the GNU General Public
 License." and was added by Ted in 1993, at which point GPLv2 only
 was the default Linux license.

Replace it with the usual GPLv2 only SPDX header.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220419063303.583106-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>