ipvs: add full ipv6 support to nfct

Prepare NFCT to support IPv6 for FTP:

- Do not restrict the expectation callback to PF_INET

- Split the debug messages, so that the 160-byte limitation
in IP_VS_DBG_BUF is not exceeded when printing many IPv6
addresses. This means no more than 3 addresses in one message,
i.e. 1 tuple with 2 addresses or 1 connection with 3 addresses.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>