CVE-2026-72326: net/sched: cake: reject overhead values that underflow length

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

net/sched: cake: reject overhead values that underflow length

CAKE accepts signed overhead values and stores them in an s16, but the adjusted packet length calculation uses unsigned arithmetic. A negative effective length can therefore wrap to a large value.

Such configurations make rate accounting depend on integer wraparound rather than on the packet size userspace intended to model. A static netlink lower bound is not enough because packets reaching CAKE can be smaller than any reasonable manual-overhead allowance.

Fold the signed overhead adjustment into the existing datapath MPU clamp so negative adjusted lengths are clamped before link-layer framing adjustments.