memcg: avoid oom-killing innocent task in case of use_hierarchy
task_in_mem_cgroup(), which is called by select_bad_process() to check
whether a task can be a candidate for being oom-killed from memcg's limit,
checks "curr->use_hierarchy"("curr" is the mem_cgroup the task belongs
to).
But this check return true(it's false positive) when:
<some path>/aa use_hierarchy == 0 <- hitting limit
<some path>/aa/00 use_hierarchy == 1 <- the task belongs to
This leads to killing an innocent task in aa/00. This patch is a fix for
this bug. And this patch also fixes the arg for
mem_cgroup_print_oom_info(). We should print information of mem_cgroup
which the task being killed, not current, belongs to.