exit: reparent: cleanup the changing of ->parent

exit: reparent: cleanup the changing of ->parent

1. Cosmetic, but "if (t->parent == father)" looks a bit confusing.
   We need to change t->parent if and only if t is not traced.

2. If we actually want this BUG_ON() to ensure that parent/ptrace
   match each other, then we should also take ptrace_reparented()
   case into account too.

3. Change this code to use for_each_thread() instead of deprecated
   while_each_thread().

[[email protected]: silence a bogus static checker warning]
Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Aaron Tomlin <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>,
Cc: Sterling Alexander <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Roland McGrath <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>