ipv4: Calling ->cow_metrics() now is a bug.

Nothing every writes to ipv4 metrics any longer.

PMTU is stored in rt->rt_pmtu.

Dynamic TCP metrics are stored in a special TCP metrics cache,
completely outside of the routes.

Therefore ->cow_metrics() can simply nothing more than a WARN_ON
trigger so we can catch anyone who tries to add new writes to
ipv4 route metrics.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -158,34 +158,8 @@ static void ipv4_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 
 static u32 *ipv4_cow_metrics(struct dst_entry *dst, unsigned long old)
 {
-	struct rtable *rt = (struct rtable *) dst;
-	struct inet_peer *peer;
-	u32 *p = NULL;
-
-	peer = rt_get_peer_create(rt, rt->rt_dst);
-	if (peer) {
-		u32 *old_p = __DST_METRICS_PTR(old);
-		unsigned long prev, new;
-
-		p = peer->metrics;
-		if (inet_metrics_new(peer))
-			memcpy(p, old_p, sizeof(u32) * RTAX_MAX);
-
-		new = (unsigned long) p;
-		prev = cmpxchg(&dst->_metrics, old, new);
-
-		if (prev != old) {
-			p = __DST_METRICS_PTR(prev);
-			if (prev & DST_METRICS_READ_ONLY)
-				p = NULL;
-		} else {
-			if (rt->fi) {
-				fib_info_put(rt->fi);
-				rt->fi = NULL;
-			}
-		}
-	}
-	return p;
+	WARN_ON(1);
+	return NULL;
 }
 
 static struct neighbour *ipv4_neigh_lookup(const struct dst_entry *dst,