net: rtnetlink: Don't export empty RTAX_FEATURES

Since the features bit field has bits for internal only use as well, it
may happen that the kernel exports RTAX_FEATURES attribute with zero
value which is pointless.

Fix this by making sure the attribute is added only if the exported
value is non-zero.
Signed-off-by: NPhil Sutter <phil@nwl.cc>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -704,6 +704,8 @@ int rtnetlink_put_metrics(struct sk_buff *skb, u32 *metrics)
 			} else if (i == RTAX_FEATURES - 1) {
 				u32 user_features = metrics[i] & RTAX_FEATURE_MASK;
 
+				if (!user_features)
+					continue;
 				BUILD_BUG_ON(RTAX_FEATURE_MASK & DST_FEATURE_MASK);
 				if (nla_put_u32(skb, i + 1, user_features))
 					goto nla_put_failure;