CVE-2026-98177: drm/amdkfd: Avoid integer underflow in EOP ring size calculation.

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

drm/amdkfd: Avoid integer underflow in EOP ring size calculation.

The low 6 bits of cphqdeopcontrol store the base-2 logarithm of the EOP ring size. This was calculated as

orderbase2(q->eopringbuffersize / 4) - 1

But orderbase2 can in theory return 0, so this could underflow (although in practice the ring buffer size cannot be less than 4096).

Change this to

orderbase2(q->eopringbuffersize / 8)

using properties of logarithms.

Also add to the above comment to make the mathematics more clear.

(cherry picked from commit f0f43fcf8b2b3a924cad9444340921c96ed5f634)