Re: Fix parallel vacuum buffer usage reporting

I've done some additional tests to validate the reported numbers. Using
pg_statio, it's possible to get the minimum number of block hits (Full
script attached).

-- Save block hits before vacuum
SELECT pg_stat_force_next_flush();
SELECT heap_blks_hit, idx_blks_hit FROM pg_statio_all_tables where
relname='vestat' \gset
vacuum (verbose, index_cleanup on) vestat;
-- Check the difference
SELECT pg_stat_force_next_flush();
SELECT heap_blks_hit - :heap_blks_hit as delta_heap_hit,
idx_blks_hit - :idx_blks_hit as delta_idx_hit,
heap_blks_hit - :heap_blks_hit + idx_blks_hit - :idx_blks_hit as sum
FROM pg_statio_all_tables where relname='vestat';

Output:
...
buffer usage: 14676 hits, 0 misses, 667 dirtied
buffer usage (new): 16081 hits, 0 misses, 667 dirtied
...
-[ RECORD 1 ]--+------
delta_heap_hit | 9747
delta_idx_hit | 6325
sum | 16072

From pg_statio, we had 16072 blocks for the relation + indexes.
Pre-patch, we are under reporting with 14676.
Post-patch, we have 16081. The 9 additional block hits come from vacuum
accessing catalog tables like pg_class or pg_class_oid_index.