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 <[email protected]>
Cc: Aaron Tomlin <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Kay Sievers <[email protected]>
Cc: Lennart Poettering <[email protected]>
Cc: Sterling Alexander <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>