IPVS: Convert __ip_vs_svc_get() and __ip_vs_fwm_get()

Add support for getting services based on their address family to
__ip_vs_service_get(), __ip_vs_fwm_get() and the helper hash function
ip_vs_svc_hashkey(). Adjust the callers.

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