pagefault_out_of_memory() should use mem_cgroup_from_task(current) and
then call mem_cgroup_out_of_memory() when it's non-NULL.
select_bad_process() will return ERR_PTR(-1UL) if there is an already oom
killed task attached to the memcg, so we can use that to avoid the
panic_on_oom. The setting of that sysctl doesn't imply that we can't scan
the tasklist, it simply means we can't kill anything as a result of an
oom.
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