ipvs: explicitly forbid ipv6 service/dest creation if ipv6 mod is disabled

When creating a new ipvs service, ipv6 addresses are always accepted
if CONFIG_IP_VS_IPV6 is enabled. On dest creation the address family
is not explicitly checked.

This allows the user-space to configure ipvs services even if the
system is booted with ipv6.disable=1. On specific configuration, ipvs
can try to call ipv6 routing code at setup time, causing the kernel to
oops due to fib6_rules_ops being NULL.

This change addresses the issue adding a check for the ipv6
module being enabled while validating ipv6 service operations and
adding the same validation for dest operations.

According to git history, this issue is apparently present since
the introduction of ipv6 support, and the oops can be triggered
since commit 09571c7a ("IPVS: Add function to determine
if IPv6 address is local")

Fixes: 09571c7a ("IPVS: Add function to determine if IPv6 address is local")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>