CVE-2026-80539: drm/amdgpu: disallow multiple FENCE chunks in one submit

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

drm/amdgpu: disallow multiple FENCE chunks in one submit

amdgpucspass1() dispatches on chunkid once per chunk without rejecting repeated ids. p->ufbo is a single-slot field, so a submission carrying two AMDGPUCHUNKIDFENCE chunks runs amdgpucsp1userfence() twice, and the second run overwrites p->ufbo with a freshly referenced BO without dropping the reference taken by the first.

amdgpucsparserfini() only unrefs the final p->ufbo, so every FENCE chunk but the last leaks a BO reference. The leaked BO outlives handle close and process exit.

Reject duplicate FENCE chunks the same way commit fec5f8e8c6bc ("drm/amdgpu: disallow multiple BOHANDLES chunks in one submit") did for p->bolist.

(cherry picked from commit 665b1fc2a1845206408f9a2c6da67101789edb82)