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

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 
Reviewed-by: Noah Misch 
Discussion: https://postgr.es/m/1f6b50a7-38ef-4d87-8246-786d39f46ab9@iki.fi