kernel - Fix panic in sysctl_kern_proc()

* sysctl_kern_proc() loops through ncpus and moves the thread to each cpu
  in turn in order to access its local thread list.

* Fix a panic where the function does not return on the same cpu it was
  called on.  The userland scheduler expects threads to return to usermode
  on the same cpu they left usermode on and is responsible for moving the
  thread to another cpu (for userland scheduling purposes) itself.