fs/proc: do_task_stat: use __for_each_thread()

[ Upstream commit 7904e53e

 ]

do/while_each_thread should be avoided when possible.

Link: https://lkml.kernel.org/r/20230909164501.GA11581@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: 7601df80

 ("fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats")
Signed-off-by: Sasha Levin <sashal@kernel.org>