stapdyn: Limit functions searches to the stap module

stapdyn: Limit functions searches to the stap module

This is slightly more efficient because we already know which object we
expect to have the functions, so we don't need to search the whole app.

* stapdyn/stapdyn.cxx (call_inferior_function): Take the BPatch_object
  in which we're searching for functions as a parameter.
  (instrument_uprobe_target, instrument_uprobes): Ditto.
  (dynamic_library_callback): Pass the stap dso from global.
  (main): Save the stap dso into a global, and pass it as needed.