The 3GPP specs are quite strange when it comes to how an IPv6 address
or rather prefix is assigned to an IPv6 PDP context. The designated
method for allocating the IPv6 address via the PDP EUA (End User
Address) Information Element in the GTP signalling plane is *not*
used to allocate the address/prefix. Instead, the EUA is used to
allocate an "interface identifier" to the MS, which it the uses
to derive its link-local source address to send a router solicitation.
The GGSN subsequently answers witha router advertisement, advertising
a single/64 prefix, whihcthe MS then uses to generate it's real IPv6
source address for subsequent communication.
Change-Id: Icddf7d30e01d76a4784bcef5787b36f52f703a9f
