oom, oom_reaper: allow to reap mm shared by the kthreads

oom, oom_reaper: allow to reap mm shared by the kthreads

oom reaper was skipped for an mm which is shared with the kernel thread
(aka use_mm()).  The primary concern was that such a kthread might want to
read from the userspace memory and see zero page as a result of the oom
reaper action.  This is no longer a problem after "mm: make sure that
kthreads will not refault oom reaped memory" because any attempt to fault
in when the MMF_UNSTABLE is set will result in SIGBUS and so the target
user should see an error.  This means that we can finally allow oom reaper
also to tasks which share their mm with kthreads.

Link: http://lkml.kernel.org/r/1472119394-11342-10-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>