cryptoloop: add a deprecation warning

cryptoloop: add a deprecation warning

Support for cryptoloop has been officially marked broken and deprecated
in favor of dm-crypt (which supports the same broken algorithms if
needed) in Linux 2.6.4 (released in March 2004), and support for it has
been entirely removed from losetup in util-linux 2.23 (released in April
2013).  Add a warning and a deprecation schedule.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210827163250.255325-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>