netfilter: fix compilation of masquerading without IP_NF_TARGET_MASQUERADE

 CONFIG_NF_NAT_MASQUERADE_IPV6=m
 # CONFIG_IP6_NF_TARGET_MASQUERADE is not set

results in:

net/ipv6/netfilter/nf_nat_masquerade_ipv6.c: In function ‘nf_nat_masquerade_ipv6’:
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:41:14: error: ‘struct nf_conn_nat’ has no member named ‘masq_index’
  nfct_nat(ct)->masq_index = out->ifindex;
              ^
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c: In function ‘device_cmp’:
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:61:12: error: ‘const struct nf_conn_nat’ has no member named ‘masq_index’
  return nat->masq_index == (int)(long)ifindex;
            ^
net/ipv6/netfilter/nf_nat_masquerade_ipv6.c:62:1: warning: control
reaches end of non-void function [-Wreturn-type]
 }
 ^
make[3]: *** [net/ipv6/netfilter/nf_nat_masquerade_ipv6.o] Error 1

Fix this by using the new NF_NAT_MASQUERADE_IPV4 and _IPV6 symbols
in include/net/netfilter/nf_nat.h.
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_nat.h

@@ -32,10 +32,8 @@ struct nf_conn_nat {
 	struct hlist_node bysource;
 	struct nf_conn *ct;
 	union nf_conntrack_nat_help help;
-#if defined(CONFIG_IP_NF_TARGET_MASQUERADE) || \
-    defined(CONFIG_IP_NF_TARGET_MASQUERADE_MODULE) || \
-    defined(CONFIG_IP6_NF_TARGET_MASQUERADE) || \
-    defined(CONFIG_IP6_NF_TARGET_MASQUERADE_MODULE)
+#if IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV4) || \
+    IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV6)
 	int masq_index;
 #endif
 };
@@ -68,8 +66,8 @@ static inline bool nf_nat_oif_changed(unsigned int hooknum,
 				      struct nf_conn_nat *nat,
 				      const struct net_device *out)
 {
-#if IS_ENABLED(CONFIG_IP_NF_TARGET_MASQUERADE) || \
-    IS_ENABLED(CONFIG_IP6_NF_TARGET_MASQUERADE)
+#if IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV4) || \
+    IS_ENABLED(CONFIG_NF_NAT_MASQUERADE_IPV6)
 	return nat->masq_index && hooknum == NF_INET_POST_ROUTING &&
 	       CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL &&
 	       nat->masq_index != out->ifindex;