exit: reparent: s/while_each_thread/for_each_thread/ in find_new_reaper()

Change find_new_reaper() to use for_each_thread() instead of deprecated
while_each_thread().  We do not bother to check "thread != father" in the
1st loop, we can rely on PF_EXITING check.

Note: this means the minor behavioural change: for_each_thread() starts
from the group leader.  But this should be fine, nobody should make any
assumption about do_wait(__WNOTHREAD) when it comes to reparented tasks.
And this can avoid the pointless reparenting to a short-living thread
While zombie leaders are not that common.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -473,8 +473,7 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 	struct pid_namespace *pid_ns = task_active_pid_ns(father);
 	struct task_struct *thread;
 
-	thread = father;
-	while_each_thread(father, thread) {
+	for_each_thread(father, thread) {
 		if (thread->flags & PF_EXITING)
 			continue;
 		if (unlikely(pid_ns->child_reaper == father))
@@ -511,11 +510,10 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 				break;
 			if (!reaper->signal->is_child_subreaper)
 				continue;
-			thread = reaper;
-			do {
+			for_each_thread(reaper, thread) {
 				if (!(thread->flags & PF_EXITING))
 					return thread;
-			} while_each_thread(reaper, thread);
+			}
 		}
 	}