Workaround the problem introduced by combination of r162200 and r215687.

r162200 delays provider orphanization until all running requests complete,
to workaround broken orphan() method implementation in some classes.
r215687 removes persistent periodic (10Hz) event thread wake ups.
Together these changes can indefinitely delay orphanization until some
other event wake up the event thread. One consequence of this is inability
of CAM to destroy device disconnected when busy and, as consequence, create
new one after reconnection.

While the best solution would be to revert r162200, it is not easy, as
some classes still look broken in that way. Instead conditionally wake up
event thread if there are some providers waiting for orphanization.

MFC after:	1 week

sys/geom/geom_event.c

@@ -294,7 +294,7 @@ g_run_events()
 		} else {
 			g_topology_unlock();
 			msleep(&g_wait_event, &g_eventlock, PRIBIO | PDROP,
-			    "-", 0);
+			    "-", TAILQ_EMPTY(&g_doorstep) ? 0 : hz / 10);
 		}
 	}
 	/* NOTREACHED */