Revert the GinMaxItemSize calculation so that we fit 3 tuples per page.

Commit 36a35c55 changed the divisor from 3 to 6, for no apparent reason.
Reducing GinMaxItemSize like that created a dump/reload hazard: loading a
9.3 database to 9.4 might fail with "index row size XXX exceeds maximum 1352
for index ..." error. Revert the change.

While we're at it, make the calculation slightly more accurate. It used to
divide the available space on page by three, then subtract
sizeof(ItemIdData), and finally round down. That's not totally accurate; the
item pointers for the three items are packed tight right after the page
header, but there is alignment padding after the item pointers. Change the
calculation to reflect that, like BTMaxItemSize does. I tested this with
different block sizes on systems with 4- and 8-byte alignment, and the value
after the final MAXALIGN_DOWN was the same with both methods on all
configurations. So this does not make any difference currently, but let's be
tidy.

Also add a comment explaining what the macro does.

This fixes bug #12292 reported by Robert Thaler. Backpatch to 9.4, where the
bug was introduced.

src/include/access/gin_private.h

@@ -226,10 +226,18 @@ typedef signed char GinNullCategory;
 #define GinGetPosting(itup)			((Pointer) ((char*)(itup) + GinGetPostingOffset(itup)))
 #define GinItupIsCompressed(itup)	(GinItemPointerGetBlockNumber(&(itup)->t_tid) & GIN_ITUP_COMPRESSED)
 
+/*
+ * Maximum size of an item on entry tree page. Make sure that we fit at least
+ * three items on each page. (On regular B-tree indexes, we must fit at least
+ * three items: two data items and the "high key". In GIN entry tree, we don't
+ * currently store the high key explicitly, we just use the rightmost item on
+ * the page, so it would actually be enough to fit two items.)
+ */
 #define GinMaxItemSize \
 	Min(INDEX_SIZE_MASK, \
-		MAXALIGN_DOWN(((BLCKSZ - SizeOfPageHeaderData -					\
-						MAXALIGN(sizeof(GinPageOpaqueData))) / 6 - sizeof(ItemIdData))))
+		MAXALIGN_DOWN(((BLCKSZ - \
+						MAXALIGN(SizeOfPageHeaderData + 3 * sizeof(ItemIdData)) - \
+						MAXALIGN(sizeof(GinPageOpaqueData))) / 3)))
 
 /*
  * Access macros for non-leaf entry tuples