loop: fix ioctl calls using compat_loop_info

Support for cryptoloop was deleted in commit 47e9624 ("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: 47e9624 ("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>

Author: Carlos Llamas

drivers/block/loop.c

@@ -1592,6 +1592,7 @@ struct compat_loop_info {
 	compat_ulong_t	lo_inode;       /* ioctl r/o */
 	compat_dev_t	lo_rdevice;     /* ioctl r/o */
 	compat_int_t	lo_offset;
+	compat_int_t	lo_encrypt_type;        /* obsolete, ignored */
 	compat_int_t	lo_encrypt_key_size;    /* ioctl w/o */
 	compat_int_t	lo_flags;       /* ioctl r/o */
 	char		lo_name[LO_NAME_SIZE];

include/uapi/linux/loop.h

@@ -45,7 +45,7 @@ struct loop_info {
 	unsigned long	   lo_inode; 		/* ioctl r/o */
 	__kernel_old_dev_t lo_rdevice; 		/* ioctl r/o */
 	int		   lo_offset;
-	int		   lo_encrypt_type;
+	int		   lo_encrypt_type;		/* obsolete, ignored */
 	int		   lo_encrypt_key_size; 	/* ioctl w/o */
 	int		   lo_flags;
 	char		   lo_name[LO_NAME_SIZE];
@@ -61,7 +61,7 @@ struct loop_info64 {
 	__u64		   lo_offset;
 	__u64		   lo_sizelimit;/* bytes, 0 == max available */
 	__u32		   lo_number;			/* ioctl r/o */
-	__u32		   lo_encrypt_type;
+	__u32		   lo_encrypt_type;		/* obsolete, ignored */
 	__u32		   lo_encrypt_key_size;		/* ioctl w/o */
 	__u32		   lo_flags;
 	__u8		   lo_file_name[LO_NAME_SIZE];