If you know the ID of a buffer that recently held a block that you would
like to pin, this function can be used check if it's still there. It
can be used to avoid a second lookup in the buffer mapping table after
PrefetchBuffer() reports a cache hit.
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq=AovOddfHpA@mail.gmail.com
2f27f8c511