net: allow to kill a task which waits net_mutex in copy_new_ns

net_mutex can be locked for a long time. It may be because many
namespaces are being destroyed or many processes decide to create
a network namespace.

Both these operations are heavy, so it is better to have an ability to
kill a process which is waiting net_mutex.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -379,7 +379,14 @@ struct net *copy_net_ns(unsigned long flags,
 
 	get_user_ns(user_ns);
 
-	mutex_lock(&net_mutex);
+	rv = mutex_lock_killable(&net_mutex);
+	if (rv < 0) {
+		net_free(net);
+		dec_net_namespaces(ucounts);
+		put_user_ns(user_ns);
+		return ERR_PTR(rv);
+	}
+
 	net->ucounts = ucounts;
 	rv = setup_net(net, user_ns);
 	if (rv == 0) {