Rework locking logic to avoid the following coverity warning.

** CID 1591966:  Concurrent data access violations  (MISSING_LOCK)
/src/detect-engine-loader.c: 475 in DetectLoadersSync()

    474                     SCCtrlMutexLock(loader->tv->ctrl_mutex);
    >>>     CID 1591966:  Concurrent data access violations  (MISSING_LOCK)
    >>>     Accessing "loader->tv" without holding lock "DetectLoaderControl_.m". Elsewhere, "DetectLoaderControl_.tv" is written to with "DetectLoaderControl_.m" held 1 out of 1 times (1 of these accesses strongly imply that it is necessary).
    475                     pthread_cond_broadcast(loader->tv->ctrl_cond);
    476                     SCCtrlMutexUnlock(loader->tv->ctrl_mutex);

The warning itself is harmless.