Remove dubious micro-optimization in ckpt_buforder_comparator().

It seems incorrect to assume that the list of CkptSortItems can never
contain duplicate page numbers: concurrent activity could result in some
page getting dropped from a low-numbered buffer and later loaded into a
high-numbered buffer while BufferSync is scanning the buffer pool.
If that happened, the comparator would give self-inconsistent results,
potentially confusing qsort().  Saving one comparison step is not worth
possibly getting the sort wrong.

So far as I can tell, nothing would actually go wrong given our current
implementation of qsort().  It might get a bit slower than expected
if there were a large number of duplicates of one value, but that's
surely a probability-epsilon case.  Still, the comment is wrong,
and if we ever switched to another sort implementation it might be
less forgiving.

In passing, avoid casting away const-ness of the argument pointers;
I've not seen any compiler complaints from that, but it seems likely
that some compilers would not like it.

Back-patch to 9.6 where this code came in, just in case I've underestimated
the possible consequences.

Discussion: https://postgr.es/m/18437.1515607610@sss.pgh.pa.us

src/backend/storage/buffer/bufmgr.c

@@ -4064,8 +4064,8 @@ local_buffer_write_error_callback(void *arg)
 static int
 rnode_comparator(const void *p1, const void *p2)
 {
-	RelFileNode n1 = *(RelFileNode *) p1;
-	RelFileNode n2 = *(RelFileNode *) p2;
+	RelFileNode n1 = *(const RelFileNode *) p1;
+	RelFileNode n2 = *(const RelFileNode *) p2;
 
 	if (n1.relNode < n2.relNode)
 		return -1;
@@ -4174,8 +4174,8 @@ buffertag_comparator(const void *a, const void *b)
 static int
 ckpt_buforder_comparator(const void *pa, const void *pb)
 {
-	const CkptSortItem *a = (CkptSortItem *) pa;
-	const CkptSortItem *b = (CkptSortItem *) pb;
+	const CkptSortItem *a = (const CkptSortItem *) pa;
+	const CkptSortItem *b = (const CkptSortItem *) pb;
 
 	/* compare tablespace */
 	if (a->tsId < b->tsId)
@@ -4195,8 +4195,10 @@ ckpt_buforder_comparator(const void *pa, const void *pb)
 	/* compare block number */
 	else if (a->blockNum < b->blockNum)
 		return -1;
-	else						/* should not be the same block ... */
+	else if (a->blockNum > b->blockNum)
 		return 1;
+	/* equal page IDs are unlikely, but not impossible */
+	return 0;
 }
 
 /*