pgsql: Make inherited TRUNCATE perform access permission checks on pare

Make inherited TRUNCATE perform access permission checks on parent table only.

Previously, TRUNCATE command through a parent table checked the
permissions on not only the parent table but also the children tables
inherited from it. This was a bug and inherited queries should perform
access permission checks on the parent table only. This commit fixes
that bug.

Back-patch to all supported branches.

Author: Amit Langote
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CAHGQGwFHdSvifhJE+-GSNqUHSfbiKxaeQQ7HGcYz6SC2n_oDcg@mail.gmail.com

Branch
------
REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a5b652f3a0110f81d2433855ab6bec136f92db83

Modified Files
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src/backend/commands/tablecmds.c | 70 +++++++++++++++++++++++---------
src/test/regress/expected/privileges.out | 21 ++++++++++
src/test/regress/sql/privileges.sql | 14 +++++++
3 files changed, 86 insertions(+), 19 deletions(-)