signal: zap_pid_ns_processes: s/SEND_SIG_NOINFO/SEND_SIG_FORCED/

Change zap_pid_ns_processes() to use SEND_SIG_FORCED, it looks more
clear compared to SEND_SIG_NOINFO which relies on from_ancestor_ns logic
send_signal().

It is also more efficient if we need to kill a lot of tasks because it
doesn't alloc sigqueue.

While at it, add the __fatal_signal_pending(task) check as a minor
optimization.
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>

kernel/pid_namespace.c

@@ -168,13 +168,9 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	while (nr > 0) {
 		rcu_read_lock();
 
-		/*
-		 * Any nested-container's init processes won't ignore the
-		 * SEND_SIG_NOINFO signal, see send_signal()->si_fromuser().
-		 */
 		task = pid_task(find_vpid(nr), PIDTYPE_PID);
-		if (task)
-			send_sig_info(SIGKILL, SEND_SIG_NOINFO, task);
+		if (task && !__fatal_signal_pending(task))
+			send_sig_info(SIGKILL, SEND_SIG_FORCED, task);
 
 		rcu_read_unlock();