gtp: use IPv6 address /64 prefix for UE/MS

Harald Welte reports that according to 3GPP TS 29.060:

    PDN Connection: the association between a MS represented by one IPv4
address and/or one IPv6 prefix and a PDN represented by an APN.

this clearly states that IPv4 is a single address while IPv6 is a single prefix.

Then, 3GPP TS 29.061, Section 11.2.1.3:

    For APNs that are configured for IPv6 address allocation, the GGSN/P-GW
shall only use the Prefix part of the IPv6 address for forwarding of mobile
terminated IP packets. The size of the prefix shall be according to the maximum
prefix length for a global IPv6 address as specified in the IPv6 Addressing
Architecture, see RFC 4291 [82].

RFC 4291 section 2.5.4 states

    All Global Unicast addresses other than those that start with binary 000
have a 64-bit interface ID field (i.e., n + m = 64) ...

3GPP TS 29.61 Section 11.2.1.3.2a:

    In the procedure in the cases of using GTP-based S5/S8, P-GW acts as an
access router, and allocates to a UE a globally unique /64 IPv6 prefix if the
PLMN allocates the prefix.

Therefore, compare IPv6 address /64 prefix only since MS/UE is not a single
address like in the IPv4 case.

Reject IPv6 address with EADDRNOTAVAIL if it lower 64 bits of the IPv6 address
from the control plane are set.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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