[6X] pg_dump: set check constraints

[kalensk]

Commit a452dba introduced a regression that removed setting nchecks on tblinfo which are the number of check constraints. Since it was never set it defaulted to zero causing no table check constraints to be dumped.

This issue was discovered by the gpupgrade upgradeable pg_upgrade acceptance tests.

Test Pipeline: https://cm.ci.gpdb.pivotal.io/teams/main/pipelines/dumpCheckConstraints

[kalensk]

Hmmm just noticed that icw_planner_centos6 is consistently failing (other icw jobs are fine) on the binary_swap test for pg_dumpall_current with:

--- \/tmp\/build\/e18b2f02\/gpdb_src\/src\/test\/binary_swap\/expected\/pg_dumpall_current\.out	2023-01-09 23:51:10.937248398 +0000
+++ \/tmp\/build\/e18b2f02\/gpdb_src\/src\/test\/binary_swap\/results\/pg_dumpall_current\.out	2023-01-09 23:51:10.937248398 +0000
@@ -1,4 +1,6 @@
 GP_IGNORE: formatted by atmsort.pm
 \! pg_dumpall -f dump_current.sql
+pg_dump: finding table check constraints
+pg_dump: finding table check constraints
 \! echo $?
 0

That line comes from pg_dump.c::getTableAttrs() which is guarded by the corresponding condition:

if (!dataOnly && checkoids->len > 2)

Will need to fix whatever is going on before merging.