Revert broken "statement with no effect" warning fix

Revert broken "statement with no effect" warning fix

It may shut up gcc, but it also incorrectly changes the semantics of the
smp_call_function() helpers.

You can fix the warning other ways if you are interested (create another
inline function that takes no arguments and returns zero), but
preferably gcc just shouldn't complain about unused return values from
statement expressions in the first place.