libipv6.c: replace usage of struct in6_addr.s6_addr16 with .s6_addr.

struct in6_addr.s6_addr16 is not standard [1]. The only standard member of
this structure is uint8_t s6_addr[16].

[1] netinet/in.h - Internet address family
    http://pubs.opengroup.org/onlinepubs/009695299/basedefs/netinet/in.h.html
libipv6.c: replace usage of struct in6_addr.s6_addr16 with .s6_addr.

struct in6_addr.s6_addr16 is not standard [1]. The only standard member of
this structure is uint8_t s6_addr[16].

[1] netinet/in.h - Internet address family
    http://pubs.opengroup.org/onlinepubs/009695299/basedefs/netinet/in.h.html