Signed overflow optimization hazards in the kernel

True enough!

However, the Linux kernel's code can be legitimately used in any GPLv2 project, including those that might run on systems with non-twos-complement signed integer arithmetic. This sharing of code among compatibly licensed projects is a very good thing, in my view.

Which means in this case, where there is a solution that meets the C standard, and which loses nothing by doing so (at least on x86 and Power), it only makes sense to use that C-standard solution.