Call to_msg_fill_raw when defined to_msg_fill is NULL

netem qdisc uses to_msg_fill_raw to build netlink packet. So far, this
was not called from anywhere.

Example creating a netem qdisc before and after the change:

[root@testbox libnl]# NLCB=debug ./netem
-- Debug: Sent Message:
--------------------------   BEGIN NETLINK MESSAGE ---------------------------
  [NETLINK HEADER] 16 octets
    .nlmsg_len = 48
    .type = 36 <route/qdisc::new>
    .flags = 1029 <REQUEST,ACK,ATOMIC>
    .seq = 1376894544
    .port = 17877
  [PAYLOAD] 20 octets
    00 00 00 00 02 00 00 00 00 00 01 80 01 00 01 00 ................
    00 00 00 00                                     ....
  [ATTR 01] 6 octets
    6e 65 74 65 6d 00                               netem.
  [PADDING] 2 octets
    00 00                                           ..
---------------------------  END NETLINK MESSAGE ---------------------------
-- Debug: Received Message:
--------------------------   BEGIN NETLINK MESSAGE ---------------------------
  [NETLINK HEADER] 16 octets
    .nlmsg_len = 68
    .type = 2 <ERROR>
    .flags = 0 <>
    .seq = 1376894544
    .port = 17877
  [ERRORMSG] 20 octets
    .error = -22 "Invalid argument"
  [ORIGINAL MESSAGE] 16 octets
    .nlmsg_len = 16
    .type = 36 <0x24>
    .flags = 1029 <REQUEST,ACK,ATOMIC>
    .seq = 1376894544
    .port = 17877
---------------------------  END NETLINK MESSAGE ---------------------------
-- Error received: Invalid argument
-- Original message: type=0x24 length=48 flags=<REQUEST,ACK,ATOMIC>
sequence-nr=1376894544 pid=17877
RC -7
[root@testbox libnl]# NLCB=debug ./netem
-- Debug: Sent Message:
--------------------------   BEGIN NETLINK MESSAGE ---------------------------
  [NETLINK HEADER] 16 octets
    .nlmsg_len = 76
    .type = 36 <route/qdisc::new>
    .flags = 1029 <REQUEST,ACK,ATOMIC>
    .seq = 1376895147
    .port = 26541
  [PAYLOAD] 20 octets
    00 00 00 00 02 00 00 00 00 00 01 80 01 00 01 00 ................
    00 00 00 00                                     ....
  [ATTR 01] 6 octets
    6e 65 74 65 6d 00                               netem.
  [PADDING] 2 octets
    00 00                                           ..
  [ATTR 02] 24 octets
    00 00 00 00 a0 86 01 00 00 00 00 80 00 00 00 00 ................
    00 00 00 00 00 00 00 00                         ........
---------------------------  END NETLINK MESSAGE ---------------------------
-- Debug: Received Message:
--------------------------   BEGIN NETLINK MESSAGE ---------------------------
  [NETLINK HEADER] 16 octets
    .nlmsg_len = 36
    .type = 2 <ERROR>
    .flags = 0 <>
    .seq = 1376895147
    .port = 26541
  [ERRORMSG] 20 octets
    .error = 0 "Success"
  [ORIGINAL MESSAGE] 16 octets
    .nlmsg_len = 16
    .type = 36 <0x24>
    .flags = 1029 <REQUEST,ACK,ATOMIC>
    .seq = 1376895147
    .port = 26541
---------------------------  END NETLINK MESSAGE  ---------------------------
RC 0
[root@testbox libnl]# tc qdisc show
qdisc htb 1: dev eth0 root refcnt 2 r2q 10 default 0 direct_packets_stat 21115
qdisc netem 8001: dev eth0 parent 1:1 limit 100000 loss 50%
qdisc htb 1: dev eth1 root refcnt 2 r2q 10 default 0 direct_packets_stat 0