drm/amdgpu: fix double free err_addr pointer warnings

In amdgpu_umc_bad_page_polling_timeout, the amdgpu_umc_handle_bad_pages
will be run many times so that double free err_addr in some special case.
So set the err_addr to NULL to avoid the warnings.
Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -179,6 +179,7 @@ void amdgpu_umc_handle_bad_pages(struct amdgpu_device *adev,
 	}
 
 	kfree(err_data->err_addr);
+	err_data->err_addr = NULL;
 
 	mutex_unlock(&con->page_retirement_lock);
 }