bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

As pg_flush_data() doesn't do anything with fsync disabled, there's no point
in tracking the buffer for writeback. Arguably the better fix would be to
change pg_flush_data() to flush data even with fsync off, but that's a
behavioral change, whereas this is just a small optimization.

Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Noah Misch <[email protected]>
Discussion: https://postgr.es/m/[email protected]

Author: anarazel

src/backend/storage/buffer/bufmgr.c

@@ -5899,7 +5899,12 @@ ScheduleBufferTagForWriteback(WritebackContext *wb_context, IOContext io_context
 {
 	PendingWriteback *pending;
 
-	if (io_direct_flags & IO_DIRECT_DATA)
+	/*
+	 * As pg_flush_data() doesn't do anything with fsync disabled, there's no
+	 * point in tracking in that case.
+	 */
+	if (io_direct_flags & IO_DIRECT_DATA ||
+		!enableFsync)
 		return;
 
 	/*