net: decrement memcg jump label when limit, not usage, is changed

The logic of the current code is that whenever we destroy
a cgroup that had its limit set (set meaning different than
maximum), we should decrement the jump_label counter.
Otherwise we assume it was never incremented.

But what the code actually does is test for RES_USAGE
instead of RES_LIMIT. Usage being different than maximum
is likely to be true most of the time.

The effect of this is that the key must become negative,
and since the jump_label test says:

        !!atomic_read(&key->enabled);

we'll have jump_labels still on when no one else is
using this functionality.
Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_memcontrol.c

@@ -108,7 +108,7 @@ void tcp_destroy_cgroup(struct cgroup *cgrp, struct cgroup_subsys *ss)
 	tcp = tcp_from_cgproto(cg_proto);
 	percpu_counter_destroy(&tcp->tcp_sockets_allocated);
 
-	val = res_counter_read_u64(&tcp->tcp_memory_allocated, RES_USAGE);
+	val = res_counter_read_u64(&tcp->tcp_memory_allocated, RES_LIMIT);
 
 	if (val != RESOURCE_MAX)
 		jump_label_dec(&memcg_socket_limit_enabled);