cpuset: fix the return value of cpuset_write_u64()

Writing to this file always returns -ENODEV:

  # echo 1 > cpuset.memory_pressure_enabled
  -bash: echo: write error: No such device

Signed-off-by: Li Zefan <lizefan@huawei.com>
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index e565778..010a008 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1608,11 +1608,13 @@
 {
 	struct cpuset *cs = cgroup_cs(cgrp);
 	cpuset_filetype_t type = cft->private;
-	int retval = -ENODEV;
+	int retval = 0;
 
 	mutex_lock(&cpuset_mutex);
-	if (!is_cpuset_online(cs))
+	if (!is_cpuset_online(cs)) {
+		retval = -ENODEV;
 		goto out_unlock;
+	}
 
 	switch (type) {
 	case FILE_CPU_EXCLUSIVE: