Fix parameters order for relation_copy_for_cluster

Hi,

When attempting to implement a new table access method, I discovered that
relation_copy_for_cluster() has the following declaration:


    void        (*relation_copy_for_cluster) (Relation NewTable,
                                              Relation OldTable,
                                              Relation OldIndex,
                                              bool use_sort,
                                              TransactionId OldestXmin,
                                              TransactionId *xid_cutoff,
                                              MultiXactId *multi_cutoff,
                                              double *num_tuples,
                                              double *tups_vacuumed,
                                              double *tups_recently_dead);

It claims that the first parameter is a new table, and the second one is an
old table.  However, the table_relation_copy_for_cluster() uses the first
parameter as the old table, and the second as a new table, see below:

static inline void
table_relation_copy_for_cluster(Relation OldTable, Relation NewTable,
                                Relation OldIndex,
                                bool use_sort,
                                TransactionId OldestXmin,
                                TransactionId *xid_cutoff,
                                MultiXactId *multi_cutoff,
                                double *num_tuples,
                                double *tups_vacuumed,
                                double *tups_recently_dead)
{
    OldTable->rd_tableam->relation_copy_for_cluster(OldTable, NewTable, OldIndex,
                                                    use_sort, OldestXmin,
                                                    xid_cutoff, multi_cutoff,
                                                    num_tuples, tups_vacuumed,
                                                    tups_recently_dead);
}

It's a bit confusing, so attach a patch to fix this.

--
Regards,
Japin Li