Protect against torn pages when deleting GIN list pages.

To-be-deleted list pages contain no useful information, as they are being
deleted, but we must still protect the writes from being torn by a crash
after a partial write. To do that, re-initialize the pages on WAL replay.

Jeff Janes caught this with a test program to test partial writes.
Backpatch to all supported versions.

src/backend/access/gin/ginxlog.c

@@ -884,25 +884,25 @@ ginRedoDeleteListPages(XLogRecPtr lsn, XLogRecord *record)
 	 * cannot get past a reader that is on, or due to visit, any page we are
 	 * going to delete.  New incoming readers will block behind our metapage
 	 * lock and then see a fully updated page list.
+	 *
+	 * No full-page images are taken of the deleted pages. Instead, they are
+	 * re-initialized as empty, deleted pages. Their right-links don't need to
+	 * be preserved, because no new readers can see the pages, as explained
+	 * above.
 	 */
 	for (i = 0; i < data->ndeleted; i++)
 	{
-		Buffer		buffer = XLogReadBuffer(data->node, data->toDelete[i], false);
+		Buffer		buffer;
+		Page		page;
 
-		if (BufferIsValid(buffer))
-		{
-			Page		page = BufferGetPage(buffer);
-
-			if (lsn > PageGetLSN(page))
-			{
-				GinPageGetOpaque(page)->flags = GIN_DELETED;
+		buffer = XLogReadBuffer(data->node, data->toDelete[i], true);
+		page = BufferGetPage(buffer);
+		GinInitBuffer(buffer, GIN_DELETED);
 
-				PageSetLSN(page, lsn);
-				MarkBufferDirty(buffer);
-			}
+		PageSetLSN(page, lsn);
+		MarkBufferDirty(buffer);
 
-			UnlockReleaseBuffer(buffer);
-		}
+		UnlockReleaseBuffer(buffer);
 	}
 	UnlockReleaseBuffer(metabuffer);
 }