[IPV6]: Fix IPV6_UNICAST_HOPS getsockopt().

> Relevant standard (RFC 3493) notes:
>
>    The IPV6_UNICAST_HOPS option may be used with getsockopt() to
>    determine the hop limit value that the system will use for subsequent
>    unicast packets sent via that socket.
>
> I don't reckon -1 could be the hop limit value.

-1 means un-initialized.

> IMHO, the value from
> case 1 (if socket is connected to some destination), otherwise case 2
> (if bound to a scope interface) or ultimately the default hop limit
> ought to be returned instead, as it will be most often correct, while
> the current behavior is always wrong, unless setsockopt() has been used
> first. I don't if some people may think doing a route lookup in
> getsockopt might be overly expensive, but at least the two other cases
> should be ok, particularly the last one.

The following patch seems to work for me, but this code has behaved this
way for a while, so don't know if it will break any existing apps.
Signed-off-by: NBrian Haley <brian.haley@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -978,12 +978,27 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case IPV6_UNICAST_HOPS:
-		val = np->hop_limit;
-		break;
-
 	case IPV6_MULTICAST_HOPS:
-		val = np->mcast_hops;
+	{
+		struct dst_entry *dst;
+
+		if (optname == IPV6_UNICAST_HOPS)
+			val = np->hop_limit;
+		else
+			val = np->mcast_hops;
+
+		dst = sk_dst_get(sk);
+		if (dst) {
+			if (val < 0)
+				val = dst_metric(dst, RTAX_HOPLIMIT);
+			if (val < 0)
+				val = ipv6_get_hoplimit(dst->dev);
+			dst_release(dst);
+		}
+		if (val < 0)
+			val = ipv6_devconf.hop_limit;
 		break;
+	}
 
 	case IPV6_MULTICAST_LOOP:
 		val = np->mc_loop;