Remove unneeded argument from _bt_getstackbuf().

_bt_getstackbuf() is called at exactly two points following commit
efada2b8 (one call site is concerned with page splits, while the
other is concerned with page deletion).  The parent buffer returned by
_bt_getstackbuf() is write-locked in both cases.  Remove the 'access'
argument and make _bt_getstackbuf() assume that callers require a
write-lock.

src/backend/access/nbtree/nbtinsert.c

@@ -1886,7 +1886,7 @@ _bt_insert_parent(Relation rel,
 		 * 05/27/97
 		 */
 		stack->bts_btentry = bknum;
-		pbuf = _bt_getstackbuf(rel, stack, BT_WRITE);
+		pbuf = _bt_getstackbuf(rel, stack);
 
 		/*
 		 * Now we can unlock the right child. The left child will be unlocked
@@ -1976,10 +1976,11 @@ _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack)
  *
  *		Adjusts bts_blkno & bts_offset if changed.
  *
- *		Returns InvalidBuffer if item not found (should not happen).
+ *		Returns write-locked buffer, or InvalidBuffer if item not found
+ *		(should not happen).
  */
 Buffer
-_bt_getstackbuf(Relation rel, BTStack stack, int access)
+_bt_getstackbuf(Relation rel, BTStack stack)
 {
 	BlockNumber blkno;
 	OffsetNumber start;
@@ -1993,11 +1994,11 @@ _bt_getstackbuf(Relation rel, BTStack stack, int access)
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, BT_WRITE);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
-		if (access == BT_WRITE && P_INCOMPLETE_SPLIT(opaque))
+		if (P_INCOMPLETE_SPLIT(opaque))
 		{
 			_bt_finish_split(rel, buf, stack->bts_parent);
 			continue;

src/backend/access/nbtree/nbtpage.c

@@ -1153,7 +1153,7 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
 	 * if needed)
 	 */
 	stack->bts_btentry = child;
-	pbuf = _bt_getstackbuf(rel, stack, BT_WRITE);
+	pbuf = _bt_getstackbuf(rel, stack);
 	if (pbuf == InvalidBuffer)
 		elog(ERROR, "failed to re-find parent key in index \"%s\" for deletion target page %u",
 			 RelationGetRelationName(rel), child);

src/include/access/nbtree.h

@@ -528,7 +528,7 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
  */
 extern bool _bt_doinsert(Relation rel, IndexTuple itup,
 			 IndexUniqueCheck checkUnique, Relation heapRel);
-extern Buffer _bt_getstackbuf(Relation rel, BTStack stack, int access);
+extern Buffer _bt_getstackbuf(Relation rel, BTStack stack);
 extern void _bt_finish_split(Relation rel, Buffer bbuf, BTStack stack);
 
 /*