drm/amdgpu: set metadata pointer to NULL after freeing.

commit 0092d3e upstream.

Without this there was a double free of the metadata,
which ended up freeing the fd table for me here, and taking
out the machine more often than not.

I reproduced with X.org + modesetting DDX + latest llvm/mesa,
also required using dri3.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -540,6 +540,7 @@ int amdgpu_bo_set_metadata (struct amdgpu_bo *bo, void *metadata,
  if (!metadata_size) {
  if (bo->metadata_size) {
  kfree(bo->metadata);
+ bo->metadata = NULL;
  bo->metadata_size = 0;
  }
  return 0;