Fix panic with net.fibs tunable set in loader.conf.

Fix by removing forgotten CTLFLAG_RWTUN flag from the sysctl,
 loader variable will be read later in vnet_rtables_init().

Reported by:	mav

sys/net/route/route_tables.c

@@ -140,7 +140,7 @@ sysctl_fibs(SYSCTL_HANDLER_ARGS)
 	return (error);
 }
 SYSCTL_PROC(_net, OID_AUTO, fibs,
-    CTLFLAG_VNET | CTLTYPE_U32 | CTLFLAG_RWTUN | CTLFLAG_MPSAFE, NULL, 0,
+    CTLFLAG_VNET | CTLTYPE_U32 | CTLFLAG_MPSAFE, NULL, 0,
     &sysctl_fibs, "IU",
     "set number of fibs");