BRIN: fix bug in xlog backup block counting

The code that generates the BRIN_XLOG_UPDATE removes the buffer
reference when the page that's target for the updated tuple is freshly
initialized.  This is a pretty usual optimization, but was breaking the
case where the revmap buffer, which is referenced in the same WAL
record, is getting a backup block: the replay code was using backup
block index 1, which is not valid when the update target buffer gets
pruned; the revmap buffer gets assigned 0 instead.  Make sure to use the
correct backup block index for revmap when replaying.

Bug reported by Fujii Masao.

src/backend/access/brin/brin_xlog.c

@@ -60,9 +60,11 @@ brin_xlog_insert_update(XLogRecPtr lsn, XLogRecord *record,
 	 */
 	if (record->xl_info & XLOG_BRIN_INIT_PAGE)
 	{
-		XLogReadBufferForRedoExtended(lsn, record, 0,
-									  xlrec->node, MAIN_FORKNUM, blkno,
-									  RBM_ZERO, false, &buffer);
+		/*
+		 * No full-page image here.  Don't try to read it, because there
+		 * might be one for the revmap buffer, below.
+		 */
+		buffer = XLogReadBuffer(xlrec->node, blkno, true);
 		page = BufferGetPage(buffer);
 		brin_page_init(page, BRIN_PAGETYPE_REGULAR);
 		action = BLK_NEEDS_REDO;
@@ -97,7 +99,9 @@ brin_xlog_insert_update(XLogRecPtr lsn, XLogRecord *record,
 		UnlockReleaseBuffer(buffer);
 
 	/* update the revmap */
-	action = XLogReadBufferForRedo(lsn, record, 1, xlrec->node,
+	action = XLogReadBufferForRedo(lsn, record,
+								   record->xl_info & XLOG_BRIN_INIT_PAGE ? 0 : 1,
+								   xlrec->node,
 								   xlrec->revmapBlk, &buffer);
 	if (action == BLK_NEEDS_REDO)
 	{