All of the other code thinks that the 8th parameter is the number of
blocks, but this declaration thinks that it's the ending block number.
Repair this inconsistency.

Patch by me, reviewed by Andres Freund.

Discussion: http://postgr.es/m/CA+TgmoY49ManQWnJtiwkuytXBkmyTuDFqb74Pr4Zn2Nq9TuNBQ@mail.gmail.com