pgsql: Skip foreign tablespaces when running pg_checksums/pg_verify_che

Skip foreign tablespaces when running pg_checksums/pg_verify_checksums

Attempting to use pg_checksums (pg_verify_checksums in 11) on a data
folder which includes tablespace paths used across multiple major
versions would cause pg_checksums to scan all directories present in
pg_tblspc, and not only marked with TABLESPACE_VERSION_DIRECTORY.  This
could lead to failures when for example running sanity checks on an
upgraded instance with --check.  Even worse, it was possible to rewrite
on-disk pages with --enable for a cluster potentially online.

This commit makes pg_checksums skip any directories not named
TABLESPACE_VERSION_DIRECTORY, similarly to what is done for base
backups.

Reported-by: Michael Banck
Author: Michael Banck, Bernd Helmle
Discussion: https://postgr.es/m/62031974fd8e941dd8351fbc8c7eff60d59c5338.camel@credativ.de
backpatch-through: 11

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/428a2609ef64b69d709418a50e192ff11a2643f1

Modified Files
--------------
src/bin/pg_checksums/pg_checksums.c   | 47 ++++++++++++++++++++++++++++++++++-
src/bin/pg_checksums/t/002_actions.pl |  9 ++++++-
2 files changed, 54 insertions(+), 2 deletions(-)