pgsql: Retain locks after marking the partitioned index valid

Retain locks after marking the partitioned index valid

Commit 8b08f7d4820f (pg11-era), for no apparent reason, made
validatePartitionedIndex() release the locks on the parent partitioned
index and table when marking the parent index valid after marking the
index partition as valid. This leads to race condition, which
particularly manifest when restoring from pg_dump in parallel mode, for
instance causing restore of FKs to fail if the PK that they depend on is
being concurrently validated. Fix by retaining those locks until
transaction commit, as we normally do everywhere in DDL code.

Per buildfarm failures.

Author: Andrey Borodin <x4mmm(at)yandex-team(dot)ru> (in absentia)
Diagnosed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Backpatch-through: 14
Discussion: https://postgr.es/m/l4joqljvbou26unhplkdj7m5olxn7rsy2loqlcta44nie5k3bs@zudmva7han7g

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1735ad557d3701a43989921be727a833646dcf89

Modified Files
--------------
src/backend/commands/tablecmds.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)