From Ingo

- performance optimization: do not kill threads in the same thread group
  as the OOM-ing thread. (it's still necessery to scan over every thread
  though, as it's possible to have CLONE_VM threads in a different thread
  group - we do not want those to escape the OOM-kill.)

- to not let newly created child threads slip out of the group-kill. Note
  that the 2.4 kernel's OOM handler has the same problem, and it could be
  the reason why forkbombs occasionally slip out of the OOM kill.