Nest: Prevent withdraws from propagation back to source protocol
The earlier fix loosen conditions for not running filters on old route when deciding about route propagation to a protocol to avoid issues with ghost routes in some race conditions. Unfortunately, the fix also caused back-propagation of withdraws. For regular updates, back-propagation is prevented in import_control hooks, but these are not called on withdraws. For them, import_control hooks are called on old routes instead, changing (old, NULL) notification to (NULL, NULL), which is ignored. By not calling export processing in some cases, the withdraw is not ignored and is back-propagated. This patch fixes that by contract conditions so the earlier fix is not applied to back-propagated updates.
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