Commit fc49e24f added an input argument after the existing output
argument.  Flip those.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20180708182345.imdgovmkffgtihhk@alvherre.pgsql