hash: Refactor bucket squeeze code.

In preparation for adding write-ahead logging to hash indexes,
refactor _hash_freeovflpage and _hash_squeezebucket so that all
related page modifications happen in a single section of code.  The
previous coding assumed that it would be fine to move tuples one at a
time, and also that the various operations involved in freeing an
overflow page didn't necessarily all need to be done together, all
of which is true if you don't care about write-ahead logging.

Amit Kapila, with slight changes by me.

src/backend/storage/page/bufpage.c

@@ -597,6 +597,33 @@ PageGetFreeSpace(Page page)
 	return (Size) space;
 }
 
+/*
+ * PageGetFreeSpaceForMultipleTuples
+ *		Returns the size of the free (allocatable) space on a page,
+ *		reduced by the space needed for multiple new line pointers.
+ *
+ * Note: this should usually only be used on index pages.  Use
+ * PageGetHeapFreeSpace on heap pages.
+ */
+Size
+PageGetFreeSpaceForMultipleTuples(Page page, int ntups)
+{
+	int			space;
+
+	/*
+	 * Use signed arithmetic here so that we behave sensibly if pd_lower >
+	 * pd_upper.
+	 */
+	space = (int) ((PageHeader) page)->pd_upper -
+		(int) ((PageHeader) page)->pd_lower;
+
+	if (space < (int) (ntups * sizeof(ItemIdData)))
+		return 0;
+	space -= ntups * sizeof(ItemIdData);
+
+	return (Size) space;
+}
+
 /*
  * PageGetExactFreeSpace
  *		Returns the size of the free (allocatable) space on a page,