sd-bus: style nitpick node_vtable_get_userdata()

It's confusing to use a single void* to store data with two different
types, i.e. a userdata value which is safe to pass to ->find(), and a
userdata value which identifies the found object.

Name the latter `found_u`.  This naming treats (!c->find) as a degenerate
case.  (I.e. at that point, we know the object has already been found :).