heap pruning: Only call BufferGetBlockNumber() once.

BufferGetBlockNumber() is not that cheap and obviously cannot change during
one heap_prune_page(), so only call it once. We might be able to do better and
pass the block number from the caller, but that'd be a larger change...

Discussion: https://postgr.es/m/20211211045710.ljtuu4gfloh754rs@alap3.anarazel.de

src/backend/access/heap/pruneheap.c

@@ -269,6 +269,7 @@ heap_page_prune(Relation relation, Buffer buffer,
 {
 	int			ndeleted = 0;
 	Page		page = BufferGetPage(buffer);
+	BlockNumber blockno = BufferGetBlockNumber(buffer);
 	OffsetNumber offnum,
 				maxoff;
 	PruneState	prstate;
@@ -335,7 +336,7 @@ heap_page_prune(Relation relation, Buffer buffer,
 		htup = (HeapTupleHeader) PageGetItem(page, itemid);
 		tup.t_data = htup;
 		tup.t_len = ItemIdGetLength(itemid);
-		ItemPointerSet(&(tup.t_self), BufferGetBlockNumber(buffer), offnum);
+		ItemPointerSet(&(tup.t_self), blockno, offnum);
 
 		/*
 		 * Set the offset number so that we can display it along with any