kernel - Fix rare allproc scan vs p_ucred race

kernel - Fix rare allproc scan vs p_ucred race

* This race can occur because p->p_ucred can change out from under
  an allproc scan when the allproc scan is filtering based on
  credentials.

* Access p->p_ucred via the per-process spinlock (p->p_spin).  Also
  maintain a  cache of the last ucred during the loop in order to
  avoid having to spin-lock every process.

* Add missing spinlock around p->p_ucred = NULL in exit1().  This
  is also only applicable to races against allproc scans since
  p_token is held during exit1().

Reported-by: mjg_