Delay struct net freeing while there's a sysfs instance refering to it

* new refcount in struct net, controlling actual freeing of the memory
	* new method in kobj_ns_type_operations (->drop_ns())
	* ->current_ns() semantics change - it's supposed to be followed by
corresponding ->drop_ns().  For struct net in case of CONFIG_NET_NS it bumps
the new refcount; net_drop_ns() decrements it and calls net_free() if the
last reference has been dropped.  Method renamed to ->grab_current_ns().
	* old net_free() callers call net_drop_ns() instead.
	* sysfs_exit_ns() is gone, along with a large part of callchain
leading to it; now that the references stored in ->ns[...] stay valid we
do not need to hunt them down and replace them with NULL.  That fixes
problems in sysfs_lookup() and sysfs_readdir(), along with getting rid
of sb->s_instances abuse.

	Note that struct net *shutdown* logics has not changed - net_cleanup()
is called exactly when it used to be called.  The only thing postponed by
having a sysfs instance refering to that struct net is actual freeing of
memory occupied by struct net.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/net/net_namespace.h

@@ -35,8 +35,11 @@ struct netns_ipvs;
 #define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
 
 struct net {
+	atomic_t		passive;	/* To decided when the network
+						 * namespace should be freed.
+						 */
 	atomic_t		count;		/* To decided when the network
-						 *  namespace should be freed.
+						 *  namespace should be shut down.
 						 */
 #ifdef NETNS_REFCNT_DEBUG
 	atomic_t		use_count;	/* To track references we
@@ -154,6 +157,9 @@ int net_eq(const struct net *net1, const struct net *net2)
 {
 	return net1 == net2;
 }
+
+extern void net_drop_ns(void *);
+
 #else
 
 static inline struct net *get_net(struct net *net)
@@ -175,6 +181,8 @@ int net_eq(const struct net *net1, const struct net *net2)
 {
 	return 1;
 }
+
+#define net_drop_ns NULL
 #endif