update ivi_xmit.c (interface id padding)

As per the latest revision of the "Mapping of Address and Port using
Translation (MAP-T)"(draft-ietf-softwire-map-t-04):

-------------------------------------
5.5. The IPv6 Interface Identifier
   The Interface identifier format of a MAP node is described below.

   |        128-n-o-s bits            |
   | 16 bits|    32 bits     | 16 bits|
   +--------+----------------+--------+
   |   0    |  IPv4 address  |  PSID  |
   +--------+----------------+--------+
                 Figure 8

   In the case of an IPv4 prefix, the IPv4 address field is right-padded
   with zeros up to 32 bits.  The k-bit PSID is zero left-padded to
   create a 16 bit field.  For an IPv4 prefix or a complete IPv4
   address, the PSID field is zero.

   If the End-user IPv6 prefix length is larger than 64, the most
   significant parts of the interface identifier is overwritten by the
   prefix.
-------------------------------------

The current version of hte tool apparently is using the older revision
of the RFC, which specifies the 8bit padding so the current patch fixes it.