netem: Fail cleanly if user input is wrong

(Resending patch since it looks like my earlier mail did not make it to
netdev).

netem reordering requires that the delay parameter be given. Currently, if no
delay is given, tc prints the error message but still installs the qdisc. Fix
this by printing the usage and failing cleanly.
Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>

tc/q_netem.c

@@ -426,6 +426,8 @@ static int netem_parse_opt(struct qdisc_util *qu, int argc, char **argv,
 	if (reorder.probability) {
 		if (opt.latency == 0) {
 			fprintf(stderr, "reordering not possible without specifying some delay\n");
+			explain();
+			return -1;
 		}
 		if (opt.gap == 0)
 			opt.gap = 1;