pgsql: Report single-page checksum failures in pg_stat_database

Report single-page checksum failures in pg_stat_database

Base backups reported checksum failures to pg_stat_database only for
files with more than one failing page. Commit 6b9e875f728 placed the
report inside the block emitting the per-file summary WARNING, which
was skipped for a single failure. As a result, a backup failing on
files with one corrupted page each left checksum_failures untouched.

To fix, emit the per-file summary and the pgstat report for any
non-zero failure count. The end-of-backup total WARNING had the same
off-by-one and is now also emitted for a single failure.

Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAN4CZFN+Bi6XmaH8zOdMWjoycYFx9nKtOr+dzQf0o-UQ+Rdqmw@mail.gmail.com
Backpatch-through: 14

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/340f2ac626586d35b13c546609571474491d6332

Modified Files
--------------
src/backend/backup/basebackup.c | 13 ++++++-------
src/bin/pg_basebackup/t/010_pg_basebackup.pl | 14 ++++++++++++++
2 files changed, 20 insertions(+), 7 deletions(-)