[IPV6]: Fix pmtu check conditions in two spots.

net/ipv6/route.c

@@ -554,6 +554,8 @@ static void ip6_rt_update_pmtu(struct dst_entry *dst, u32 mtu)
 
 	if (mtu < dst_pmtu(dst) && rt6->rt6i_dst.plen == 128) {
 		rt6->rt6i_flags |= RTF_MODIFIED;
+		if (mtu < IPV6_MIN_MTU)
+			mtu = IPV6_MIN_MTU;
 		dst->metrics[RTAX_MTU-1] = mtu;
 	}
 }

net/ipv6/xfrm6_policy.c

@@ -236,10 +236,10 @@ static void xfrm6_update_pmtu(struct dst_entry *dst, u32 mtu)
 {
 	struct dst_entry *path = dst->path;
 
-	if (mtu >= 1280 && mtu < dst_pmtu(dst))
-		return;
-
-	path->ops->update_pmtu(path, mtu);
+	if (mtu >= IPV6_MIN_MTU && mtu < dst_pmtu(dst))
+		path->ops->update_pmtu(path, mtu);
+	
+	return;
 }
 
 struct dst_ops xfrm6_dst_ops = {