libnetlink: add size argument to rtnl_talk

There have been several instances where response from kernel
has overrun the stack buffer from the caller. Avoid future problems
by passing a size argument.

Also drop the unused peer and group arguments to rtnl_talk.
diff --git a/ip/link_vti.c b/ip/link_vti.c
index 59ac4c4..f3fea33 100644
--- a/ip/link_vti.c
+++ b/ip/link_vti.c
@@ -71,7 +71,7 @@
 		req.i.ifi_family = preferred_family;
 		req.i.ifi_index = ifi->ifi_index;
 
-		if (rtnl_talk(&rth, &req.n, 0, 0, &req.n) < 0) {
+		if (rtnl_talk(&rth, &req.n, &req.n, sizeof(req)) < 0) {
 get_failed:
 			fprintf(stderr,
 				"Failed to get existing tunnel info.\n");