ip: return correct exit code on route failure

If kernel complains about ip route request, exit status should be
2 not 1.

This fixes regression introduced by:
commit 42ecedd4ba
Author: Roopa Prabhu <roopa@cumulusnetworks.com>
Date:   Tue Mar 17 19:26:32 2015 -0700

    fix ip -force -batch to continue on errors

ip/iproute.c

@@ -1164,7 +1164,7 @@ static int iproute_modify(int cmd, unsigned flags, int argc, char **argv)
 		req.r.rtm_family = AF_INET;
 
 	if (rtnl_talk(&rth, &req.n, 0, 0, NULL) < 0)
-		return -1;
+		return -2;
 
 	return 0;
 }