[IPV6]: Don't update ADVMSS on routes where the MTU is not also updated

The ADVMSS value was incorrectly updated for ALL routes when the MTU
is updated because it's outside the effect of the if statement's
condition.
Signed-off-by: NSimon Arlott <simon@fire.lp0.eu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1983,9 +1983,10 @@ static int rt6_mtu_change_route(struct rt6_info *rt, void *p_arg)
 	    !dst_metric_locked(&rt->u.dst, RTAX_MTU) &&
 	    (dst_mtu(&rt->u.dst) > arg->mtu ||
 	     (dst_mtu(&rt->u.dst) < arg->mtu &&
-	      dst_mtu(&rt->u.dst) == idev->cnf.mtu6)))
+	      dst_mtu(&rt->u.dst) == idev->cnf.mtu6))) {
 		rt->u.dst.metrics[RTAX_MTU-1] = arg->mtu;
-	rt->u.dst.metrics[RTAX_ADVMSS-1] = ipv6_advmss(arg->mtu);
+		rt->u.dst.metrics[RTAX_ADVMSS-1] = ipv6_advmss(arg->mtu);
+	}
 	return 0;
 }