cdrom: Make device operations read-only

Since function tables are a common target for attackers, it's best to keep
them in read-only memory. As such, this makes the CDROM device ops tables
const. This drops additionally n_minors, since it isn't used meaningfully,
and sets the only user of cdrom_dummy_generic_packet explicitly so the
variables can all be const.

Inspired by similar changes in grsecurity/PaX.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/ide/ide-cd.c

@@ -1166,7 +1166,7 @@ void ide_cdrom_update_speed(ide_drive_t *drive, u8 *buf)
 	 CDC_CD_RW | CDC_DVD | CDC_DVD_R | CDC_DVD_RAM | CDC_GENERIC_PACKET | \
 	 CDC_MO_DRIVE | CDC_MRW | CDC_MRW_W | CDC_RAM)
 
-static struct cdrom_device_ops ide_cdrom_dops = {
+static const struct cdrom_device_ops ide_cdrom_dops = {
 	.open			= ide_cdrom_open_real,
 	.release		= ide_cdrom_release_real,
 	.drive_status		= ide_cdrom_drive_status,