netfilter: don't pull include/linux/netfilter.h from netns headers

This pulls the full hook netfilter definitions from all those that include
net_namespace.h.

Instead let's just include the bare minimum required in the new
linux/netfilter_defs.h file, and use it from the netfilter netns header files.

I also needed to include in.h and in6.h from linux/netfilter.h otherwise we hit
this compilation error:

In file included from include/linux/netfilter_defs.h:4:0,
from include/net/netns/netfilter.h:4,
from include/net/net_namespace.h:22,
from include/linux/netdevice.h:43,
from net/netfilter/nfnetlink_queue_core.c:23:
include/uapi/linux/netfilter.h:76:17: error: field ‘in’ has incomplete type struct in_addr in;

And also explicit include linux/netfilter.h in several spots.

Signed-off-by: Pablo Neira Ayuso <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

include/linux/netfilter.h

···
 #include <linux/wait.h>
 #include <linux/list.h>
 #include <linux/static_key.h>
-#include <uapi/linux/netfilter.h>
+#include <linux/netfilter_defs.h>
+
 #ifdef CONFIG_NETFILTER
 static inline int NF_DROP_GETERR(int verdict)
 {
···
 }
 
 int netfilter_init(void);
-
-/* Largest hook number + 1 */
-#define NF_MAX_HOOKS 8
 
 struct sk_buff;
 

include/linux/netfilter_defs.h

···
+#ifndef __LINUX_NETFILTER_CORE_H_
+#define __LINUX_NETFILTER_CORE_H_
+
+#include <uapi/linux/netfilter.h>
+
+/* Largest hook number + 1, see uapi/linux/netfilter_decnet.h */
+#define NF_MAX_HOOKS 8
+
+#endif

include/net/netns/netfilter.h

···
 #ifndef __NETNS_NETFILTER_H
 #define __NETNS_NETFILTER_H
 
-#include <linux/netfilter.h>
+#include <linux/netfilter_defs.h>
 
 struct proc_dir_entry;
 struct nf_logger;

include/net/netns/x_tables.h

···
 #define __NETNS_X_TABLES_H
 
 #include <linux/list.h>
-#include <linux/netfilter.h>
+#include <linux/netfilter_defs.h>
 
 struct ebt_table;
 

include/uapi/linux/netfilter.h

···
 #include <linux/types.h>
 #include <linux/compiler.h>
 #include <linux/sysctl.h>
-
+#include <linux/in.h>
+#include <linux/in6.h>
 
 /* Responses from hook functions. */
 #define NF_DROP 0

net/ipv6/output_core.c

···
 #include <net/ip6_fib.h>
 #include <net/addrconf.h>
 #include <net/secure_seq.h>
+#include <linux/netfilter.h>
 
 static u32 __ipv6_select_ident(struct net *net, u32 hashrnd,
 			       const struct in6_addr *dst,