Make blkreftable API use size_t/ssize_t consistently

Make blkreftable API use size_t/ssize_t consistently

It was using int for the input and output length, where size_t or
ssize_t would be more appropriate.  It might not matter in practice,
but it makes the APIs consistent at all levels.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/f9aab072-0078-49e4-ab93-3b08086a4406@eisentraut.org