loop: fix ioctl calls using compat_loop_info

Support for cryptoloop was deleted in commit 47e96246 ("block:
remove support for cryptoloop and the xor transfer"), making the usage
of loop_info->lo_encrypt_type obsolete. However, this member was also
removed from the compat_loop_info definition and this breaks userspace
ioctl calls for 32-bit binaries and CONFIG_COMPAT=y.

This patch restores the compat_loop_info->lo_encrypt_type member and
marks it obsolete as well as in the uapi header definitions.

Fixes: 47e96246

 ("block: remove support for cryptoloop and the xor transfer")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220329201815.1347500-1-cmllamas@google.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>