cfm: Always initialize CCM "internal_ms_x" extension field.

Found by valgrind:

Syscall param socketcall.sendmsg(msg.msg_iov[i]) points to uninitialised
byte(s)
   at 0x42D3021: sendmsg (in /lib/libc-2.5.so)
   by 0x80E4D23: nl_sock_transact (netlink-socket.c:670)
   by 0x80D9086: dpif_linux_execute__ (dpif-linux.c:872)
   by 0x807D6AE: dpif_execute__ (dpif.c:957)
   by 0x807D6FE: dpif_execute (dpif.c:987)
   by 0x805DED9: send_packet (ofproto-dpif.c:4727)
   by 0x805F8E1: port_run_fast (ofproto-dpif.c:2441)
   by 0x8065CF6: run_fast (ofproto-dpif.c:926)
   by 0x805674F: ofproto_run_fast (ofproto.c:1148)
   by 0x804C957: bridge_run_fast (bridge.c:1980)
   by 0x8053F49: main (ovs-vswitchd.c:123)
  Address 0xbea0895c is on thread 1's stack

Bug #11797.
Signed-off-by: Ben Pfaff <blp@nicira.com>