[Oneiric CVE-2013-0871] race conditon in ptrace

[Luis Henriques]

>From the description:

 "A race conditon in ptrace can lead to kernel stack corruption and
  arbitrary kernel-mode code execution."

Following this email, there are 3 patches that should fix this issue for
Oneiric.

I did a lot of testing with this backport and although I'm pretty confident
that I'm not introducing any regressions, a good review would be great!