netfilter: use IS_ENABLED() macro

replace:
 #if defined(CONFIG_NF_CT_NETLINK) || defined(CONFIG_NF_CT_NETLINK_MODULE)
with
 #if IS_ENABLED(CONFIG_NF_CT_NETLINK)

replace:
 #if !defined(CONFIG_NF_NAT) && !defined(CONFIG_NF_NAT_MODULE)
with
 #if !IS_ENABLED(CONFIG_NF_NAT)

replace:
 #if !defined(CONFIG_NF_CONNTRACK) && !defined(CONFIG_NF_CONNTRACK_MODULE)
with
 #if !IS_ENABLED(CONFIG_NF_CONNTRACK)

And add missing:
 IS_ENABLED(CONFIG_NF_CT_NETLINK)

in net/ipv{4,6}/netfilter/nf_nat_l3proto_ipv{4,6}.c

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>