signal: complete_signal: use __for_each_thread()

do/while_each_thread should be avoided when possible.

Link: https://lkml.kernel.org/r/20230909164537.GA11633@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>