ip route: don't assume default route

Just print the help when "ip route del" is called without any other
arguments.

Resolves:

 * https://bugzilla.redhat.com/show_bug.cgi?id=997965

Signed-off-by: Pavel Šimerda <psimerda@redhat.com>

ip/iproute.c

@@ -975,6 +975,9 @@ static int iproute_modify(int cmd, unsigned flags, int argc, char **argv)
 		argc--; argv++;
 	}
 
+	if (!dst_ok)
+		usage();
+
 	if (d || nhs_ok)  {
 		int idx;