netfilter: nf_defrag_ipv4: fix compilation error with NF_CONNTRACK=n

As reported by Randy Dunlap <[e-mail]>, compilation
of nf_defrag_ipv4 fails with:

include/net/netfilter/nf_conntrack.h:94: error: field 'ct_general' has incomplete type
include/net/netfilter/nf_conntrack.h:178: error: 'const struct sk_buff' has no member named 'nfct'
include/net/netfilter/nf_conntrack.h:185: error: implicit declaration of function 'nf_conntrack_put'
include/net/netfilter/nf_conntrack.h:294: error: 'const struct sk_buff' has no member named 'nfct'
net/ipv4/netfilter/nf_defrag_ipv4.c:45: error: 'struct sk_buff' has no member named 'nfct'
net/ipv4/netfilter/nf_defrag_ipv4.c:46: error: 'struct sk_buff' has no member named 'nfct'

net/nf_conntrack.h must not be included with NF_CONNTRACK=n, add a
few #ifdefs. Long term the header file should be fixed to be usable
even with NF_CONNTRACK=n.

Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>

include/net/netfilter/nf_conntrack_zones.h

···
 #ifndef _NF_CONNTRACK_ZONES_H
 #define _NF_CONNTRACK_ZONES_H
 
-#include <net/netfilter/nf_conntrack_extend.h>
-
 #define NF_CT_DEFAULT_ZONE	0
+
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+#include <net/netfilter/nf_conntrack_extend.h>
 
 struct nf_conntrack_zone {
 	u16	id;
···
 	return NF_CT_DEFAULT_ZONE;
 }
 
+#endif /* CONFIG_NF_CONNTRACK || CONFIG_NF_CONNTRACK_MODULE */
 #endif /* _NF_CONNTRACK_ZONES_H */

net/ipv4/netfilter/nf_defrag_ipv4.c

···
 
 #include <linux/netfilter_bridge.h>
 #include <linux/netfilter_ipv4.h>
-#include <net/netfilter/nf_conntrack_zones.h>
 #include <net/netfilter/ipv4/nf_defrag_ipv4.h>
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
 #include <net/netfilter/nf_conntrack.h>
+#endif
+#include <net/netfilter/nf_conntrack_zones.h>
 
 /* Returns new sk_buff, or NULL */
 static int nf_ct_ipv4_gather_frags(struct sk_buff *skb, u_int32_t user)
···
 {
 	u16 zone = NF_CT_DEFAULT_ZONE;
 
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
 	if (skb->nfct)
 		zone = nf_ct_zone((struct nf_conn *)skb->nfct);
+#endif
 
 #ifdef CONFIG_BRIDGE_NETFILTER
 	if (skb->nf_bridge &&