CVE-2024-26884

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

bpf: Fix hashtab overflow check on 32-bit arches

The hashtab code relies on rounduppowoftwo() to compute the number of hash buckets, and contains an overflow check by checking if the resulting value is 0. However, on 32-bit arches, the roundup code itself can overflow by doing a 32-bit left-shift of an unsigned long value, which is undefined behaviour, so it is not guaranteed to truncate neatly. This was triggered by syzbot on the DEVMAPHASH type, which contains the same check, copied from the hashtab code. So apply the same fix to hashtab, by moving the overflow check to before the roundup.