oom_kill: add rcu_read_lock() into find_lock_task_mm()

oom_kill: add rcu_read_lock() into find_lock_task_mm()

find_lock_task_mm() expects it is called under rcu or tasklist lock, but
it seems that at least oom_unkillable_task()->task_in_mem_cgroup() and
mem_cgroup_out_of_memory()->oom_badness() can call it lockless.

Perhaps we could fix the callers, but this patch simply adds rcu lock
into find_lock_task_mm().  This also allows to simplify a bit one of its
callers, oom_kill_process().

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Sergey Dyasly <[email protected]>
Cc: Sameer Nanda <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Mandeep Singh Baines <[email protected]>
Cc: "Ma, Xindong" <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: "Tu, Xiaobing" <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>