MAINTAINERS: relocate cpuset.c

Commit 201af4c0fab0 ("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>

MAINTAINERS

@@ -3579,7 +3579,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git
 S:	Maintained
 F:	Documentation/cgroup-v1/cpusets.txt
 F:	include/linux/cpuset.h
-F:	kernel/cpuset.c
+F:	kernel/cgroup/cpuset.c
 
 CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)
 M:	Johannes Weiner <hannes@cmpxchg.org>