the attached patch updates a number of items:

 - adds cleanups suggested by Christoph Hellwig: needed unlikely()
   statements, a superfluous #define and line length problems.

 - splits up the global ptrace list into per-task ptrace lists. This was
   pretty straightforward, and this makes the worst-case exit() latency
   O(nr_children).

the per-task ptrace lists unearthed a bug that the previous code did not
take care of: tasks on the ptrace list have to be correctly reparented as
well. This patch passed my stresstests as well.