Correct the scope of vmtimer in UnregisteredClasses::load_class

The vmtimer in the following is not measuring anything; its scope should be expanded to measure the entire function.

InstanceKlass* UnregisteredClasses::load_class(Symbol* name, const char* path, TRAPS) {
  assert(name != nullptr, "invariant");
  assert(DumpSharedSpaces, "this function is only used with -Xshare:dump");

  {
    PerfClassTraceTime vmtimer(ClassLoader::perf_sys_class_lookup_time(),
                               THREAD->get_thread_stat()->perf_timers_addr(),
                               PerfClassTraceTime::CLASS_LOAD);
  }