Store the deletion horizon XID for a deleted GIN page on the right page.

Commit b1071408 moved the GinPageSetDeleteXid() call to a spot where
the "page" variable was pointing to the wrong page, causing the XID
to be inserted on a page that's not being deleted, thus allowing later
GinPageIsRecyclable tests to recycle the deleted page too soon.

It might be a good idea to stop using the single "page" variable for
multiple purposes in this function.  But for the moment I just moved
the GinPageSetDeleteXid() call down beside the GinPageSetDeleted()
call, which seems like a more logical place for it anyway.

Back-patch to v11, as the faulty patch was.  (Fortunately, the bug
hasn't made it into any release yet.)

Discussion: https://postgr.es/m/21620.1581098806@sss.pgh.pa.us

src/backend/access/gin/ginvacuum.c

@@ -165,9 +165,6 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	page = BufferGetPage(lBuffer);
 	GinPageGetOpaque(page)->rightlink = rightlink;
 
-	/* For deleted page remember last xid which could knew its address */
-	GinPageSetDeleteXid(page, ReadNewTransactionId());
-
 	/* Delete downlink from parent */
 	parentPage = BufferGetPage(pBuffer);
 #ifdef USE_ASSERT_CHECKING
@@ -186,7 +183,13 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	 * we shouldn't change rightlink field to save workability of running
 	 * search scan
 	 */
+
+	/*
+	 * Mark page as deleted, and remember last xid which could know its
+	 * address.
+	 */
 	GinPageSetDeleted(page);
+	GinPageSetDeleteXid(page, ReadNewTransactionId());
 
 	MarkBufferDirty(pBuffer);
 	MarkBufferDirty(lBuffer);