block: scsi_ioctl: Avoid the use of one-element arrays

One-element arrays are being deprecated[1]. Replace the one-element array
with a simple object of type compat_caddr_t: 'compat_caddr_t unused'[2],
once it seems this field is actually never used.

Also, update struct cdrom_generic_command in UAPI by adding an
anonimous union to avoid using the one-element array _reserved_.

[1] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
[2] https://github.com/KSPP/linux/issues/86

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Build-tested-by: kernel test robot <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/scsi_ioctl.c

···
 	unsigned char	data_direction;
 	compat_int_t	quiet;
 	compat_int_t	timeout;
-	compat_caddr_t	reserved[1];
+	compat_caddr_t	unused;
 };
 #endif
 
···
 			.data_direction	= cgc32.data_direction,
 			.quiet		= cgc32.quiet,
 			.timeout	= cgc32.timeout,
-			.reserved[0]	= compat_ptr(cgc32.reserved[0]),
+			.unused		= compat_ptr(cgc32.unused),
 		};
 		memcpy(&cgc->cmd, &cgc32.cmd, CDROM_PACKET_SIZE);
 		return 0;
···
 			.data_direction	= cgc->data_direction,
 			.quiet		= cgc->quiet,
 			.timeout	= cgc->timeout,
-			.reserved[0]	= (uintptr_t)(cgc->reserved[0]),
+			.unused		= (uintptr_t)(cgc->unused),
 		};
 		memcpy(&cgc32.cmd, &cgc->cmd, CDROM_PACKET_SIZE);
 

include/uapi/linux/cdrom.h

···
 	unsigned char		data_direction;
 	int			quiet;
 	int			timeout;
-	void			__user *reserved[1];	/* unused, actually */
+	union {
+		void		__user *reserved[1];	/* unused, actually */
+		void            __user *unused;
+	};
 };
 
 /*