cdrom: rearrange last_media_change check to avoid unintentional overflow

In the Linux kernel, the following vulnerability has been resolved:

cdrom: rearrange last_media_change check to avoid unintentional overflow

When running syzkaller with the newly reintroduced signed integer wrap

sanitizer we encounter this splat:

[ 366.015950] UBSAN: signed-integer-overflow in ../drivers/cdrom/cdrom.c:2361:33

[ 366.021089] -9223372036854775808 - 346321 cannot be represented in type '__s64' (aka 'long long')

[ 366.025894] program syz-executor.4 is using a deprecated SCSI ioctl, please convert it to SG_IO

[ 366.027502] CPU: 5 PID: 28472 Comm: syz-executor.7 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1

[ 366.027512] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014

[ 366.027518] Call Trace:

[ 366.027523] <TASK>

[ 366.027533] dump_stack_lvl+0x93/0xd0

[ 366.027899] handle_overflow+0x171/0x1b0

[ 366.038787] ata1.00: invalid multi_count 32 ignored

[ 366.043924] cdrom_ioctl+0x2c3f/0x2d10

[ 366.063932] ? __pm_runtime_resume+0xe6/0x130

[ 366.071923] sr_block_ioctl+0x15d/0x1d0

[ 366.074624] ? __pfx_sr_block_ioctl+0x10/0x10

[ 366.077642] blkdev_ioctl+0x419/0x500

[ 366.080231] ? __pfx_blkdev_ioctl+0x10/0x10

...

Historically, the signed integer overflow sanitizer did not work in the

kernel due to its interaction with `-fwrapv` but this has since been

changed [1] in the newest version of Clang. It was re-enabled in the

kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow

sanitizer").

Let's rearrange the check to not perform any arithmetic, thus not

tripping the sanitizer.