[IPV6]: Update MSS even if MTU is unchanged.

This is needed because in ndisc.c, we have:

  static void ndisc_router_discovery(struct sk_buff *skb)
  {
  // ...
  	if (ndopts.nd_opts_mtu) {
  // ...
  			if (rt)
  				rt->u.dst.metrics[RTAX_MTU-1] = mtu;

  			rt6_mtu_change(skb->dev, mtu);
  // ...
  }

Since the mtu is set directly here, rt6_mtu_change_route thinks that
it is unchanged, and so it fails to update the MSS accordingly.  This
patch lets rt6_mtu_change_route still update MSS if old_mtu == new_mtu.

Signed-off-by: Jim Paris <jim@jtan.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1909,7 +1909,7 @@ static int rt6_mtu_change_route(struct rt6_info *rt, void *p_arg)
 	 */
 	if (rt->rt6i_dev == arg->dev &&
 	    !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) < arg->mtu &&
 	      dst_mtu(&rt->u.dst) == idev->cnf.mtu6))) {
 		rt->u.dst.metrics[RTAX_MTU-1] = arg->mtu;