When creating a new md, wait for geom's event queue to become empty

before returning.  Device nodes are created via the "taste" mechanism,
so this is necessary in order to make sure that devfs entries are
created before mdconfig(8) returns.

This may be a MFC candidate for 5.3.

Suggested by:	phk

sys/dev/md/md.c

@@ -735,6 +735,7 @@ mdinit(struct md_s *sc)
 	sc->pp = pp;
 	g_error_provider(pp, 0);
 	g_topology_unlock();
+	g_waitidle();
 	PICKUP_GIANT();
 }