IPVS: Add general v4/v6 helper functions / data structures

Add a struct ip_vs_iphdr for easier handling of common v4 and v6 header
fields in the same code path. ip_vs_fill_iphdr() helps to fill this struct
from an IPv4 or IPv6 header. Add further helper functions for copying and
comparing addresses.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>