Change pgstat_report_vacuum() to use Relation

Change pgstat_report_vacuum() to use Relation

This change makes pgstat_report_vacuum() more consistent with
pgstat_report_analyze(), that also uses a Relation.  This enforces a
policy that callers of this routine should open and lock the relation
whose statistics are updated before calling this routine.  We will
unlikely have a lot of callers of this routine in the tree, but it seems
like a good idea to imply this requirement in the long run.

Author: Bertrand Drouvot <[email protected]>
Suggested-by: Andres Freund <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Discussion: https://postgr.es/m/[email protected]