BUG #18087: pg_visibility: false positives in pg_check_visible

The following bug has been logged on the website:

Bug reference:      18087
Logged by:          yajun Hu
Email address:      1026592243@qq.com
PostgreSQL version: 11.21
Operating system:   CentOS7 with kernel version 5.10
Description:

hi, community partners. 
    I found a false positives when use pg_check_visible, I build postgresql
with REL_11_STABLE, run this SQL
select  pg_create_logical_replication_slot  ('test','pgoutput');
create table t( a int);
insert into t select 1;
insert into t select 1;
insert into t select 1;
vacuum t;
 select * from pg_check_visible('t'); -- output ctid, which is false
positives

I analyze this problem, guess in collect_corrupt_items, GetOldestXmin use
rel = NULL
to get alldbs oldestXmin which will influenced by replication slot and other
database
long transaction. But Vacuum get relation's database oldestXmin.

I will put my fix patch behind

Regrads, Yajun Hu