ipv6: add mtu lock check in __ip6_rt_update_pmtu

Prior to this patch, ipv6 didn't do mtu lock check in ip6_update_pmtu.
It leaded to that mtu lock doesn't really work when receiving the pkt
of ICMPV6_PKT_TOOBIG.

This patch is to add mtu lock check in __ip6_rt_update_pmtu just as ipv4
did in __ip_rt_update_pmtu.
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1364,6 +1364,9 @@ static void __ip6_rt_update_pmtu(struct dst_entry *dst, const struct sock *sk,
 	if (rt6->rt6i_flags & RTF_LOCAL)
 		return;
 
+	if (dst_metric_locked(dst, RTAX_MTU))
+		return;
+
 	dst_confirm(dst);
 	mtu = max_t(u32, mtu, IPV6_MIN_MTU);
 	if (mtu >= dst_mtu(dst))