htb: report overhead attribute

"tc class show dev ..." omits the overhead attribute for HTB.

After patch I have :

tc class add dev $DEV parent 1: classid 1:1 est 1sec 4sec htb \
    rate 12Mbit mtu 1500 quantum 1514 overhead 20

tc class show dev $DEV
class htb 1:1 root prio 0 rate 12000Kbit overhead 20 ceil 12000Kbit
burst 1500b cburst 1500b
Signed-off-by: Eric Dumazet <edumazet@google.com>

tc/q_htb.c

@@ -264,6 +264,8 @@ static int htb_print_opt(struct qdisc_util *qu, FILE *f, struct rtattr *opt)
 				fprintf(f, "quantum %d ", (int)hopt->quantum);
 		}
 	    fprintf(f, "rate %s ", sprint_rate(hopt->rate.rate, b1));
+	    if (hopt->rate.overhead)
+		fprintf(f, "overhead %u ", hopt->rate.overhead);
 	    buffer = tc_calc_xmitsize(hopt->rate.rate, hopt->buffer);
 	    fprintf(f, "ceil %s ", sprint_rate(hopt->ceil.rate, b1));
 	    cbuffer = tc_calc_xmitsize(hopt->ceil.rate, hopt->cbuffer);