netfilter: defrag: only register defrag functionality if needed

nf_defrag modules for ipv4 and ipv6 export an empty stub function.
Any module that needs the defragmentation hooks registered simply 'calls'
this empty function to create a phony module dependency -- modprobe will
then load the defrag module too.

This extends netfilter ipv4/ipv6 defragmentation modules to delay the hook
registration until the functionality is requested within a network namespace
instead of module load time for all namespaces.

Hooks are only un-registered on module unload or when a namespace that used
such defrag functionality exits.

We have to use struct net for this as the register hooks can be called
before netns initialization here from the ipv4/ipv6 conntrack module
init path.

There is no unregister functionality support, defrag will always be
active once it was requested inside a net namespace.

The reason is that defrag has impact on nft and iptables rulesets
(without defrag we might see framents).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/ipv4/nf_defrag_ipv4.h

@@ -1,6 +1,7 @@
 #ifndef _NF_DEFRAG_IPV4_H
 #define _NF_DEFRAG_IPV4_H
 
-void nf_defrag_ipv4_enable(void);
+struct net;
+int nf_defrag_ipv4_enable(struct net *);
 
 #endif /* _NF_DEFRAG_IPV4_H */

include/net/netfilter/ipv6/nf_defrag_ipv6.h

@@ -1,7 +1,8 @@
 #ifndef _NF_DEFRAG_IPV6_H
 #define _NF_DEFRAG_IPV6_H
 
-void nf_defrag_ipv6_enable(void);
+struct net;
+int nf_defrag_ipv6_enable(struct net *);
 
 int nf_ct_frag6_init(void);
 void nf_ct_frag6_cleanup(void);

include/net/netns/netfilter.h

@@ -17,5 +17,11 @@ struct netns_nf {
 	struct ctl_table_header *nf_log_dir_header;
 #endif
 	struct nf_hook_entry __rcu *hooks[NFPROTO_NUMPROTO][NF_MAX_HOOKS];
+#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4)
+	bool			defrag_ipv4;
+#endif
+#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
+	bool			defrag_ipv6;
+#endif
 };
 #endif