mm, oom: show process exiting information in __oom_kill_process()

When the OOM killer finds a victim and tryies to kill it, if the victim is
already exiting, the task mm will be NULL and no process will be killed.
But the dump_header() has been already executed, so it will be strange to
dump so much information without killing a process.  We'd better show some
helpful information to indicate why this happens.
Suggested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200721010127.17238-1-laoar.shao@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -861,6 +861,8 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
 
 	p = find_lock_task_mm(victim);
 	if (!p) {
+		pr_info("%s: OOM victim %d (%s) is already exiting. Skip killing the task\n",
+			message, task_pid_nr(victim), victim->comm);
 		put_task_struct(victim);
 		return;
 	} else if (victim != p) {