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 <pablo@netfilter.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
diff --git a/net/ipv6/output_core.c b/net/ipv6/output_core.c
index 21678ac..928a0fb 100644
--- a/net/ipv6/output_core.c
+++ b/net/ipv6/output_core.c
@@ -8,6 +8,7 @@
 #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,