net/ipv6: Convert ipv6_add_addr to struct ifa6_config
Move config parameters for adding an ipv6 address to a struct. struct
names stem from inet6_rtm_newaddr which is the modern handler for
adding an address.
Start the conversion to ifa6_config with ipv6_add_addr. This is an argument
move only; no functional change intended. Mapping of variable changes: