ip: netconf: fix overzealous error checking

The rtnetlink.sh kernel test started reporting errors after
iproute2 update. The error checking introduced by commit
under fixes is incorrect. rtnl_listen() always returns
an error, because the only way to break the loop is to
return an error from the handler, it seems.

Switch this code to using normal rtnl_talk(), instead of
the rtnl_listen() abuse. As far as I can tell the use of
rtnl_listen() was to make get and dump use common handling
but that's no longer the case, anyway.

Before:
  $ ip -6 netconf show dev lo
  inet6 lo forwarding off mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off
  $ echo $?
  2

After:
  $ ./ip/ip -6 netconf show dev lo
inet6 lo forwarding off mc_forwarding off proxy_neigh off ignore_routes_with_linkdown off
  $ echo $?
  0

Fixes: 00e8a64dac ("ip: detect errors in netconf monitor mode")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

ip/ipnetconf.c

@@ -187,16 +187,16 @@ static int do_show(int argc, char **argv)
 	ll_init_map(&rth);
 
 	if (filter.ifindex && filter.family != AF_UNSPEC) {
+		struct nlmsghdr *answer;
+
 		req.ncm.ncm_family = filter.family;
 		addattr_l(&req.n, sizeof(req), NETCONFA_IFINDEX,
 			  &filter.ifindex, sizeof(filter.ifindex));
 
-		if (rtnl_send(&rth, &req.n, req.n.nlmsg_len) < 0) {
+		if (rtnl_talk(&rth, &req.n, &answer) < 0)
-			perror("Can not send request");
-			exit(1);
-		}
-		if (rtnl_listen(&rth, print_netconf, stdout) < 0)
 			exit(2);
+
+		print_netconf2(answer, stdout);
 	} else {
 		rth.flags = RTNL_HANDLE_F_SUPPRESS_NLERR;
 dump: