drm/amdgpu: fix sched fence slab teardown

To free fences, call_rcu() is used, which calls amd_sched_fence_free()
after a grace period. During teardown, there is no guarantee all
callbacks have finished, so sched_fence_slab may be destroyed before
all fences have been freed. If we are lucky, this results in some slab
warnings, if not, we get a crash in one of rcu threads because callback
is called after amdgpu has already been unloaded.

Fix it with a rcu_barrier().

Fixes: 189e0fb763 ("drm/amdgpu: RCU protected amd_sched_fence_release")
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/scheduler/gpu_scheduler.c

@@ -645,6 +645,7 @@ void amd_sched_fini(struct amd_gpu_scheduler *sched)
 {
 	if (sched->thread)
 		kthread_stop(sched->thread);
+	rcu_barrier();
 	if (atomic_dec_and_test(&sched_fence_slab_ref))
 		kmem_cache_destroy(sched_fence_slab);
 }