Reduce the scope of volatile qualifiers

Reduce the scope of volatile qualifiers

Commit c66a7d75e652 introduced a new "cast discards ‘volatile’"
warning (-Wcast-qual) in vac_truncate_clog().

Instead of making use of unvolatize(), remove the warning by reducing the
scope of the volatile qualifier (added in commit 2d2e40e3bef) to only
2 fields.

Also do the same for vac_update_datfrozenxid(), since the intent of
commit f65ab862e3b was to prevent the same kind of race condition that
commit 2d2e40e3bef was fixing.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/aZ3a%2BV82uSfEjDmD%40ip-10-97-1-34.eu-west-3.compute.internal