exit: reparent: introduce find_alive_thread()

exit: reparent: introduce find_alive_thread()

Add the new simple helper to factor out the for_each_thread() code in
find_child_reaper() and find_new_reaper().  It can also simplify the
potential PF_EXITING -> exit_state change, plus perhaps we can change this
code to take SIGNAL_GROUP_EXIT into account.

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>