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
------
REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/928e755d22de56e998b0b76596ebda1941e0e323

Modified Files
--------------
src/backend/commands/tablecmds.c | 67 +++++++++++++++++++++++---------
src/test/regress/expected/privileges.out | 21 ++++++++++
src/test/regress/sql/privileges.sql | 14 +++++++
3 files changed, 84 insertions(+), 18 deletions(-)