orangefs: turn param response value into union

orangefs: turn param response value into union

This will support a upcoming request where two related values need to be
updated atomically.

This was done without a union in the OrangeFS server source already. Since
that will break the kernel protocol, it has been fixed there and done here
in a way that does not break the kernel protocol.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>