net: Make account struct net to memcg

The patch adds SLAB_ACCOUNT to flags of net_cachep cache,
which enables accounting of struct net memory to memcg kmem.
Since number of net_namespaces may be significant, user
want to know, how much there were consumed, and control.

Note, that we do not account net_generic to the same memcg,
where net was accounted, moreover, we don't do this at all (*).
We do not want the situation, when single memcg memory deficit
prevents us to register new pernet_operations.

(*)Even despite there is !current process accounting already
available in linux-next. See kmalloc_memcg() there for the details.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -882,7 +882,7 @@ static int __init net_ns_init(void)
 #ifdef CONFIG_NET_NS
 	net_cachep = kmem_cache_create("net_namespace", sizeof(struct net),
 					SMP_CACHE_BYTES,
-					SLAB_PANIC, NULL);
+					SLAB_PANIC|SLAB_ACCOUNT, NULL);
 
 	/* Create workqueue for cleanup */
 	netns_wq = create_singlethread_workqueue("netns");