Pull proc fix from Eric Biederman:
 "A brown paper bag slipped through my proc changes, and syzcaller
  caught it when the code ended up in your tree.

  I have opted to fix it the simplest cleanest way I know how, so there
  is no reasonable chance for the bug to repeat"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  proc: Use a dedicated lock in struct pid