drm/amdgpu: Fix use-after-free in amdgpu_cs_ioctl

[Why]
In amdgpu_cs_ioctl, amdgpu_job_free could be performed ealier if there
is -ERESTARTSYS error. In this case, job->hw_fence could be not
initialized yet. Putting hw_fence during amdgpu_job_free could lead to a
use-after-free warning.

[How]
Check if drm_sched_job_init is performed before job_free by checking
s_fence.

v2: Check hw_fence.ops instead since it could be NULL if fence is not
initialized. Reverse the condition since !=NULL check is discouraged in
kernel.
Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -162,7 +162,10 @@ void amdgpu_job_free(struct amdgpu_job *job)
 	amdgpu_sync_free(&job->sync);
 	amdgpu_sync_free(&job->sched_sync);
 
-	dma_fence_put(&job->hw_fence);
+	if (!job->hw_fence.ops)
+		kfree(job);
+	else
+		dma_fence_put(&job->hw_fence);
 }
 
 int amdgpu_job_submit(struct amdgpu_job *job, struct drm_sched_entity *entity,