netfilter: add defines for arp/decnet max hooks

The kernel already has defines for this, but they are in uapi exposed
headers.

Including these from netns.h causes build errors and also adds unneeded
dependencies on heads that we don't need.

So move these defines to netfilter_defs.h and place the uapi ones
in ifndef __KERNEL__ to keep them for userspace.

Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/linux/netfilter_defs.h

···
 /* Largest hook number + 1, see uapi/linux/netfilter_decnet.h */
 #define NF_MAX_HOOKS 8
 
+/* in/out/forward only */
+#define NF_ARP_NUMHOOKS 3
+
+/* max hook is NF_DN_ROUTE (6), also see uapi/linux/netfilter_decnet.h */
+#define NF_DN_NUMHOOKS 7
+
 #endif

include/uapi/linux/netfilter_arp.h

···
 #define NF_ARP_IN	0
 #define NF_ARP_OUT	1
 #define NF_ARP_FORWARD	2
+
+#ifndef __KERNEL__
 #define NF_ARP_NUMHOOKS	3
+#endif
 
 #endif /* __LINUX_ARP_NETFILTER_H */

include/uapi/linux/netfilter_decnet.h

···
 #define NFC_DN_IF_IN		0x0004
 /* Output device. */
 #define NFC_DN_IF_OUT		0x0008
+
+/* kernel define is in netfilter_defs.h */
+#define NF_DN_NUMHOOKS		7
 #endif /* ! __KERNEL__ */
 
 /* DECnet Hooks */
···
 #define NF_DN_HELLO		5
 /* Input Routing Packets */
 #define NF_DN_ROUTE		6
-#define NF_DN_NUMHOOKS		7
 
 enum nf_dn_hook_priorities {
 	NF_DN_PRI_FIRST = INT_MIN,