pgsql: Use PG_DATA_CHECKSUM_OFF instead of hardcoded value

Use PG_DATA_CHECKSUM_OFF instead of hardcoded value

For a long time, the online checksums patchset kept the "off" state as
literal zero without a label to be consistent with the previous coding
which only had a label for the "on" state. Later, when an "off" label
was made not all uses in the code got the memo. Fix by setting these
to PG_DATA_CHECKSUM_OFF.

While there, fix a duplicate word in a comment introduced by the same
commit.

Author: Aleksander Alekseev <aleksander(at)tigerdata(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/CAJ7c6TPRTnQFXXX1CRcYoTLXw2swtDH==uSz1MYoMKdLrKZHjA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3a37ffbc5b94c0214f1f3f1953e800654a1962f

Modified Files
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src/backend/access/transam/xlog.c | 10 +++++-----
src/backend/bootstrap/bootstrap.c | 2 +-
src/bin/pg_checksums/pg_checksums.c | 4 ++--
src/bin/pg_combinebackup/pg_combinebackup.c | 4 ++--
src/bin/pg_upgrade/controldata.c | 10 +++++-----
src/bin/pg_upgrade/file.c | 2 +-
6 files changed, 16 insertions(+), 16 deletions(-)