getrusage: use __for_each_thread()

[ Upstream commit 13b7bc60b5353371460a203df6c38ccd38ad7a3a ]

do/while_each_thread should be avoided when possible.

Plus this change allows to avoid lock_task_sighand(), we can use rcu
and/or sig->stats_lock instead.

Link: https://lkml.kernel.org/r/20230909172629.GA20454@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: f7ec1cd5cc7e ("getrusage: use sig->stats_lock rather than lock_task_sighand()")
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/sys.c

@@ -1807,10 +1807,8 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
 		r->ru_oublock += sig->oublock;
 		if (maxrss < sig->maxrss)
 			maxrss = sig->maxrss;
-		t = p;
+		__for_each_thread(sig, t)
-		do {
 			accumulate_thread_rusage(t, r);
-		} while_each_thread(p, t);
 		break;
 
 	default: