proc_oom_score: remove tasklist_lock and pid_alive()

proc_oom_score: remove tasklist_lock and pid_alive()

This was needed before to ensure that ->signal != 0 and do_each_thread()
is safe, see commit b95c35e76b29b ("oom: fix the unsafe usage of
badness() in proc_oom_score()") for details.

Today tsk->signal can't go away and for_each_thread(tsk) is always safe.

Link: http://lkml.kernel.org/r/20160608211921.GA15508@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>