android, lmk: avoid setting TIF_MEMDIE if process has already exited

TIF_MEMDIE should not be set on a process if it does not have a valid
->mm, and this is protected by task_lock().

If TIF_MEMDIE gets set after the mm has detached, and the process fails to
exit, then the oom killer will defer forever waiting for it to exit.

Make sure that the mm is still valid before setting TIF_MEMDIE by way of
mark_tsk_oom_victim().

Cc: "Arve Hjønnevåg" <arve@android.com>
Cc: Riley Andrews <riandrews@android.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>