Fix nbtree cleanup-only VACUUM stats inaccuracies.

Logic for counting heap TIDs from posting list tuples (added by commit
0d861bbb) was faulty.  It didn't count any TIDs/index tuples in the
event of no callback being set.  This meant that we incorrectly counted
no index tuples in clean-up only VACUUMs, which could lead to
pg_class.reltuples being spuriously set to 0 in affected indexes.

To fix, go back to counting items from the page in cases where there is
no callback.  This approach isn't very accurate, but it works well
enough in practice while avoiding the expense of accessing every index
tuple during cleanup-only VACUUMs.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
https://postgr.es/m/20201023174451.69e358f1@firost
Backpatch: 13-, where nbtree deduplication was introduced

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