ipv6: delete expired route in ip6_pmtu_deliver

The first big packets sent to a "low-MTU" client correctly
triggers the creation of a temporary route containing the reduced MTU.

But after the temporary route has expired, new ICMP6 "packet too big"
will be sent, rt6_pmtu_discovery will find the previous EXPIRED route
check that its mtu isn't bigger then in icmp packet and do nothing
before the temporary route will not deleted by gc.

I make the simple experiment:
while :; do
    time ( dd if=/dev/zero bs=10K count=1 | ssh hostname dd of=/dev/null ) || break;
done

The "time" reports real 0m0.197s if a temporary route isn't expired, but
it reports real 0m52.837s (!!!!) immediately after a temporare route has
expired.
Signed-off-by: NAndrey Vagin <avagin@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1565,11 +1565,16 @@ static void rt6_do_pmtu_disc(struct in6_addr *daddr, struct in6_addr *saddr,
 {
 	struct rt6_info *rt, *nrt;
 	int allfrag = 0;
-
+again:
 	rt = rt6_lookup(net, daddr, saddr, ifindex, 0);
 	if (rt == NULL)
 		return;
 
+	if (rt6_check_expired(rt)) {
+		ip6_del_rt(rt);
+		goto again;
+	}
+
 	if (pmtu >= dst_mtu(&rt->dst))
 		goto out;