cgroup: rename cgroup_iter to cgroup_task_iter

cgroup now has multiple iterators and it's quite confusing to have
something which walks over tasks of a single cgroup named cgroup_iter.
Let's rename it to cgroup_task_iter.

While at it, reformat / update comments and replace the overview
comment above the interface function decls with proper function
comments.  Such overview can be useful but function comments should be
more than enough here.

This is pure rename and doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2285319..00b055d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1800,11 +1800,11 @@
 	totalpages = mem_cgroup_get_limit(memcg) >> PAGE_SHIFT ? : 1;
 	for_each_mem_cgroup_tree(iter, memcg) {
 		struct cgroup *cgroup = iter->css.cgroup;
-		struct cgroup_iter it;
+		struct cgroup_task_iter it;
 		struct task_struct *task;
 
-		cgroup_iter_start(cgroup, &it);
-		while ((task = cgroup_iter_next(cgroup, &it))) {
+		cgroup_task_iter_start(cgroup, &it);
+		while ((task = cgroup_task_iter_next(cgroup, &it))) {
 			switch (oom_scan_process_thread(task, totalpages, NULL,
 							false)) {
 			case OOM_SCAN_SELECT:
@@ -1817,7 +1817,7 @@
 			case OOM_SCAN_CONTINUE:
 				continue;
 			case OOM_SCAN_ABORT:
-				cgroup_iter_end(cgroup, &it);
+				cgroup_task_iter_end(cgroup, &it);
 				mem_cgroup_iter_break(memcg, iter);
 				if (chosen)
 					put_task_struct(chosen);
@@ -1834,7 +1834,7 @@
 				get_task_struct(chosen);
 			}
 		}
-		cgroup_iter_end(cgroup, &it);
+		cgroup_task_iter_end(cgroup, &it);
 	}
 
 	if (!chosen)