cgroup: make cgroup_task_iter remember the cgroup being iterated

Currently all cgroup_task_iter functions require @cgrp to be passed
in, which is superflous and increases chance of usage error.  Make
cgroup_task_iter remember the cgroup being iterated and drop @cgrp
argument from next and end functions.

This patch doesn't introduce any behavior differences.

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 00b055d..5a5f4dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1804,7 +1804,7 @@
 		struct task_struct *task;
 
 		cgroup_task_iter_start(cgroup, &it);
-		while ((task = cgroup_task_iter_next(cgroup, &it))) {
+		while ((task = cgroup_task_iter_next(&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_task_iter_end(cgroup, &it);
+				cgroup_task_iter_end(&it);
 				mem_cgroup_iter_break(memcg, iter);
 				if (chosen)
 					put_task_struct(chosen);
@@ -1834,7 +1834,7 @@
 				get_task_struct(chosen);
 			}
 		}
-		cgroup_task_iter_end(cgroup, &it);
+		cgroup_task_iter_end(&it);
 	}
 
 	if (!chosen)