oom: avoid sending exiting tasks a SIGKILL

It's unnecessary to SIGKILL a task that is already PF_EXITING and can
actually cause a NULL pointer dereference of the sighand if it has already
been detached.  Instead, simply set TIF_MEMDIE so it has access to memory
reserves and can quickly exit as the comment implies.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -453,7 +453,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * its children or threads, just set TIF_MEMDIE so it can die quickly
 	 */
 	if (p->flags & PF_EXITING) {
-		__oom_kill_task(p, 0);
+		set_tsk_thread_flag(p, TIF_MEMDIE);
 		return 0;
 	}