MAINTAINERS: relocate cpuset.c

Commit 201af4c0 ("cgroup: move cgroup files under kernel/cgroup/")
have renamed kernel/cpuset.c to kernel/cgroup/cpuset.c, but forgot to
update entry of cpuset module in MAINTAINERS, fix it.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>