Nest: Improve export counter handling
One of previous workarounds for phantom route avoidance breaks export counters by expanding sending of spurious withdraws, which are send when we are not sure whether we have advertised that routes in the past. If not, then export counter is decreased, but it was not increased before, so it overflows under zero. The patch fixes that by sendung spurious withdraws, but not counting them on export counter. That may lead to error in the other direction, but that happens only as a race condition (i.e., in normal operation filters return proper values about old route export state).
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