ipv6: add fib6_has_custom_rules() helper

It wraps the namespace field with the same name, to easily
access it regardless of build options.

Suggested-by: David Ahern <dsahern@gmail.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>