signal: oom_kill_task: use SEND_SIG_FORCED instead of force_sig()

Change oom_kill_task() to use do_send_sig_info(SEND_SIG_FORCED) instead
of force_sig(SIGKILL).  With the recent changes we do not need force_ to
kill the CLONE_NEWPID tasks.

And this is more correct.  force_sig() can race with the exiting thread
even if oom_kill_task() checks p->mm != NULL, while
do_send_sig_info(group => true) kille the whole process.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -521,11 +521,11 @@ static void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 			pr_err("Kill process %d (%s) sharing same memory\n",
 				task_pid_nr(p), p->comm);
 			task_unlock(p);
-			force_sig(SIGKILL, p);
+			do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
 		}
 
 	set_tsk_thread_flag(victim, TIF_MEMDIE);
-	force_sig(SIGKILL, victim);
+	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
 }
 #undef K