ipv4: distinguish EHOSTUNREACH from the ENETUNREACH

since commit 251da41("ipv4: Cache ip_error() routes even when not forwarding."),
the counter IPSTATS_MIB_INADDRERRORS can't work correctly, because the value of
err was always set to ENETUNREACH.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1695,8 +1695,11 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	fl4.daddr = daddr;
 	fl4.saddr = saddr;
 	err = fib_lookup(net, &fl4, &res);
-	if (err != 0)
+	if (err != 0) {
+		if (!IN_DEV_FORWARD(in_dev))
+			err = -EHOSTUNREACH;
 		goto no_route;
+	}
 
 	RT_CACHE_STAT_INC(in_slow_tot);
 
@@ -1712,8 +1715,10 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 		goto local_input;
 	}
 
-	if (!IN_DEV_FORWARD(in_dev))
+	if (!IN_DEV_FORWARD(in_dev)) {
+		err = -EHOSTUNREACH;
 		goto no_route;
+	}
 	if (res.type != RTN_UNICAST)
 		goto martian_destination;