debug cgroup: remove unneeded cgroup_lock

Since we are in cgroup write handler, so the cgrp is valid, so we don't
have to hold cgroup_mutex when calling cgroup_task_count().  One similar
example is in cgroup_tasks_open().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup_debug.c

@@ -40,9 +40,7 @@ static u64 taskcount_read(struct cgroup *cont, struct cftype *cft)
 {
 	u64 count;
 
-	cgroup_lock();
 	count = cgroup_task_count(cont);
-	cgroup_unlock();
 	return count;
 }