IPVS: use ipv6_addr_copy()

It is standard to use ipv6_addr_copy() to fill in
the in6 element of a union nf_inet_addr snet.

Thanks to Julius Volz for pointing this out.

Cc: Brian Haley <brian.haley@hp.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Julius Volz <juliusv@google.com>