Fix autovacuum log output heap truncation issue.

Fix autovacuum log output heap truncation issue.

The percentage of blocks from the table value reported by autovacuum log
output (following commit 5100010ee4d) should never exceed 100% because
it describes the state of the table back when lazy_vacuum() was called.
The value could nevertheless exceed 100% in the event of heap relation
truncation.  We failed to compensate for how truncation affects
rel_pages.

Fix the faulty accounting by using the original rel_pages value instead
of the current/final rel_pages value.

Reported-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210423204306.5osfpkt2ggaedyvy@alap3.anarazel.de