fib_rules: Fix dump_rules() not to exit early

Backports of 41fc0143 ("fib_rules: fix fib rule dumps across
multiple skbs") introduced a regression in "ip rule show" - it ends up
dumping the first rule over and over and never exiting, because 3.19
and earlier are missing commit 053c095a ("netlink: make
nlmsg_end() and genlmsg_end() void"), so fib_nl_fill_rule() ends up
returning skb->len (i.e. > 0) in the success case.

Fix this by checking the return code for < 0 instead of != 0.
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/core/fib_rules.c

@@ -631,7 +631,7 @@ static int dump_rules(struct sk_buff *skb, struct netlink_callback *cb,
 		err = fib_nl_fill_rule(skb, rule, NETLINK_CB(cb->skb).portid,
 				       cb->nlh->nlmsg_seq, RTM_NEWRULE,
 				       NLM_F_MULTI, ops);
-		if (err)
+		if (err < 0)
 			break;
 skip:
 		idx++;