IPVS: Add function to determine if IPv6 address is local

Add __ip_vs_addr_is_local_v6() to find out if an IPv6 address belongs to a
local interface. Use this function to decide whether to set the
IP_VS_CONN_F_LOCALNODE flag for IPv6 destinations.

Signed-off-by: Vince Busam <vbusam@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>