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>
Merge branch 'for-3.11-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup fix from Tejun Heo:
 "This contains one patch to fix the return value of cpuset's cgroups
  interface function, which used to always return -ENODEV for the writes
  on the 'memory_pressure_enabled' file"

* 'for-3.11-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cpuset: fix the return value of cpuset_write_u64()