[NETNS]: fix net released by rcu callback

When a network namespace reference is held by a network subsystem,
and when this reference is decremented in a rcu update callback, we
must ensure that there is no more outstanding rcu update before
trying to free the network namespace.

In the normal case, the rcu_barrier is called when the network namespace
is exiting in the cleanup_net function.

But when a network namespace creation fails, and the subsystems are
undone (like the cleanup), the rcu_barrier is missing.

This patch adds the missing rcu_barrier.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -112,6 +112,8 @@ out_undo:
 		if (ops->exit)
 			ops->exit(net);
 	}
+
+	rcu_barrier();
 	goto out;
 }