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