Correctly set userid of subquery relations' child rels

Correctly set userid of subquery relations' child rels

The RelOptInfo->userid field (the user ID to check permissions as) of an
"otherrel" relation was being copied from its parent relation, which is
correct in most cases but wrong when the parent is a subquery.  In that
case, using the value from the RTEPermissionInfo of the child itself is
the appropriate thing to do.

Coming up with a test case where user-visible behavior changes proves
hard enough, so we don't add one here.

Bug introduced by a61b1f74823c, discovered by Amit while reviewing
nearby code.

Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqE0WY_AhLnGtTsY7eYebG212XWbM-D8gr2A_ToOHyCywQ@mail.gmail.com