update: Drop produce() return value and parameters work, amount

update: Drop produce() return value and parameters work, amount

Since changing *sp is safe now, we can move the update of
sp->sct_avail into produce().  This frees the return value; use it to
return the amount produced.  Drop the parameters.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>