pgsql: Close race condition between datfrozen and relfrozen updates.

Close race condition between datfrozen and relfrozen updates.

vac_update_datfrozenxid() did multiple loads of relfrozenxid and
relminmxid from buffer memory, and it assumed each would get the same
value. Not so if a concurrent vac_update_relstats() did an inplace
update. Commit 2d2e40e3befd8b9e0d2757554537345b15fa6ea2 fixed the same
kind of bug in vac_truncate_clog(). Today's bug could cause the
rel-level field and XIDs in the rel's rows to precede the db-level
field. A cluster having such values should VACUUM affected tables.
Back-patch to v12 (all supported versions).

Discussion: https://postgr.es/m/20240423003956.e7.nmisch@google.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/92685c389d5416f87553bfb80205d79d5670a695

Modified Files
--------------
src/backend/commands/vacuum.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)