fix linux-yocto-rt compile error

kernel/cgroup/cpuset.c:2959:28: error: 'cpuset_rwsem' undeclared (first use in this function); did you mean 'cpuset_mutex'?
 2959 |         percpu_down_write(&cpuset_rwsem);
      |                            ^~~~~~~~~~~~
      |                            cpuset_mutex

Signed-off-by: Li Wang <li.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>