IPVS: Allow adding IPv6 services from userspace

Allow adding IPv6 services through the genetlink interface and add checks
to see if the chosen scheduler is supported with IPv6 and whether the
supplied prefix length is sane. Make sure the service count exported via
the sockopt interface only counts IPv4 services.

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