IPVS: Add/adjust Netfilter hook functions and helpers for v6

Add Netfilter hook functions or modify existing ones, if possible, to
process IPv6 packets. Some support functions are also added/modified for
this. ip_vs_nat_icmp_v6() was already added in the patch that added the v6
xmit functions, as it is called from one of them.
Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>