netfilter: nf_conntrack_h323: Remove deprecated config check

CONFIG_NF_CONNTRACK_IPV6 has been deprecated so replace it with a check
for IPV6 instead.

Use nf_ip6_route6() instead of v6ops->route() and keep the IS_MODULE()
in nf_ipv6_ops as mentioned by Florian so that direct calls are used
when IPV6 is builtin and indirect calls are used only when IPV6 is a
module.

Fixes: a0ae2562 ("netfilter: conntrack: remove l3proto abstraction")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>