Fix dmemcache_object flush functionality

I have performed a code audit of the memcache module, the dmemcache_object function supports a $flush parameter which defaults to FALSE.

The logic if this parameter is set to true, will never actually flush the memcache instances because the $extension variable is defined later in the function by mistake.

After a quick grep of the codebase, it doesn't look like the $flush parameter is ever set to TRUE which may be why no one has reported this :)