Fix incorrect pg_stat_io output on 32-bit machines.

Fix incorrect pg_stat_io output on 32-bit machines.

pg_stat_get_io() applied TimestampTzGetDatum twice to the
stat_reset_timestamp value.  On 64-bit builds that's harmless because
TimestampTzGetDatum is a no-op, but on 32-bit builds it results in
displaying garbage in the stats_reset column of the pg_stat_io view.

Bug dates to commit a9c70b46d which introduced pg_stat_io, so
back-patch to v16 where that came in.

Bertrand Drouvot

Discussion: https://postgr.es/m/Ztrd+XcPTz1zorkg@ip-10-97-1-34.eu-west-3.compute.internal