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 sending 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).