BUG #17064: Parallel VACUUM operations cause the error "global/pg_filenode.map contains incorrect checksum"

The following bug has been logged on the website:

Bug reference:      17064
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 14beta1
Operating system:   Ubuntu 20.04
Description:

The following script:
===
for i in `seq 100`; do
createdb db$i
done

# Based on the contents of the regression test "vacuum"
echo "
CREATE TABLE pvactst (i INT);
INSERT INTO pvactst SELECT i FROM generate_series(1,10000) i;
DELETE FROM pvactst;
VACUUM pvactst;
DROP TABLE pvactst;

VACUUM FULL pg_database;
" >/tmp/vacuum.sql

for n in `seq 10`; do
  echo "iteration $n"
  for i in `seq 100`; do
    ( { for f in `seq 100`; do cat /tmp/vacuum.sql; done } | psql -d db$i )
>psql-$i.log 2>&1 &
  done
  wait
  grep -C5 FATAL psql*.log && break;
done
===
detects sporadic FATAL errors:
iteration 1
psql-56.log-DROP TABLE
psql-56.log-VACUUM
psql-56.log-CREATE TABLE
psql-56.log-INSERT 0 10000
psql-56.log-DELETE 10000
psql-56.log:FATAL:  relation mapping file "global/pg_filenode.map" contains
incorrect checksum
psql-56.log-server closed the connection unexpectedly
psql-56.log-    This probably means the server terminated abnormally
psql-56.log-    before or while processing the request.
psql-56.log-connection to server was lost

(Discovered when running multiple installcheck's for a single instance.
(Except for this failure, installcheck x 100 (with several tests excluded)
is rather stable.))

Reproduced on REL9_6_STABLE .. master.