drm/amdgpu: Fix __rcu fence pointer accesses

Building for ARCH=um with W=1 C=1 makes sparse report "incompatible
types in comparison expression (different address spaces)" warnings in
the KFD code, exposed after UML builds were enabled:

- amdgpu_amdkfd_fence.c compares the __rcu-annotated dma_fence.ops
  pointer directly in to_amdgpu_amdkfd_fence().
- amdgpu_amdkfd_gpuvm.c compares the __rcu eviction fence pointer
  directly in amdgpu_amdkfd_gpuvm_restore_process_bos().

Fixes: af3f2f5db2 ("drm/amdgpu: Remove UML build exclusion from Kconfig")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607091659.SHEscT0c-lkp@intel.com/
Cc: Harry Wentland <harry.wentland@amd.com>
Assisted-by: Copilot:Claude-Opus-4.8
Signed-off-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 764f241ad2)

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_fence.c

@@ -92,7 +92,7 @@ struct amdgpu_amdkfd_fence *to_amdgpu_amdkfd_fence(struct dma_fence *f)
 		return NULL;
 
 	fence = container_of(f, struct amdgpu_amdkfd_fence, base);
-	if (f->ops == &amdkfd_fence_ops)
+	if (rcu_access_pointer(f->ops) == &amdkfd_fence_ops)
 		return fence;
 
 	return NULL;

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c

@@ -3096,7 +3096,7 @@ int amdgpu_amdkfd_gpuvm_restore_process_bos(void *info, struct dma_fence __rcu *
 		process_info->eviction_fence = new_fence;
 		replace_eviction_fence(ef, dma_fence_get(&new_fence->base));
 	} else {
-		WARN_ONCE(*ef != &process_info->eviction_fence->base,
+		WARN_ONCE(rcu_access_pointer(*ef) != &process_info->eviction_fence->base,
 			  "KFD eviction fence doesn't match KGD process_info");
 	}