[UNIX]: Make the unix sysctl tables per-namespace

This is the core.

 * add the ctl_table_header on the struct net;
 * make the unix_sysctl_register and _unregister clone the table;
 * moves calls to them into per-net init and exit callbacks;
 * move the .data pointer in the proper place.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -11,6 +11,8 @@
 struct proc_dir_entry;
 struct net_device;
 struct sock;
+struct ctl_table_header;
+
 struct net {
 	atomic_t		count;		/* To decided when the network
 						 *  namespace should be freed.
@@ -41,6 +43,7 @@ struct net {
 
 	/* unix sockets */
 	int			sysctl_unix_max_dgram_qlen;
+	struct ctl_table_header	*unix_ctl;
 };
 
 #ifdef CONFIG_NET