drm/amdgpu: avoid double drm_exec_fini() in userq validate

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

drm/amdgpu: avoid double drm_exec_fini() in userq validate

When new_addition is true, amdgpu_userq_vm_validate() calls drm_exec_fini(&exec) before iterating over the collected HMM ranges and calling amdgpu_ttm_tt_get_user_pages().

If amdgpu_ttm_tt_get_user_pages() fails in that path, the code jumps to unlock_all and calls drm_exec_fini(&exec) a second time on the same exec object. drm_exec_fini() is not idempotent: it frees exec->objects and may also drop exec->contended and finalize the ww acquire context.

Route that error path directly to the range cleanup once exec has already been finalized.

Issue found using a prototype static analysis tool and confirmed by code review.

(cherry picked from commit 2802952e4a07306da6ebe813ff1acacc5691851a)