block/scsi-ioctl: Fix kernel-infoleak in scsi_put_cdrom_generic_arg()

scsi_put_cdrom_generic_arg() is copying uninitialized stack memory to
userspace, since the compiler may leave a 3-byte hole in the middle of
`cgc32`. Fix it by adding a padding field to `struct
compat_cdrom_generic_command`.

Cc: stable@vger.kernel.org
Fixes: f3ee6e63a9 ("compat_ioctl: move CDROM_SEND_PACKET handling into scsi")
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: syzbot+85433a479a646a064ab3@syzkaller.appspotmail.com
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/scsi_ioctl.c

@@ -651,6 +651,7 @@ struct compat_cdrom_generic_command {
 	compat_int_t	stat;
 	compat_caddr_t	sense;
 	unsigned char	data_direction;
+	unsigned char	pad[3];
 	compat_int_t	quiet;
 	compat_int_t	timeout;
 	compat_caddr_t	reserved[1];