kernel - Address excessive stall in pageout during deadlock avoidance

kernel - Address excessive stall in pageout during deadlock avoidance

* The pageout daemon uses LK_TIMELOCK to avoid deadlocking on a vnode
  that might be held locked during a pfault, which waits up to 1/10 of
  a second.

  If the vnode has a large number of pageable pages each page may go
  through the timeout.  This can result in the pageout daemon stalling
  for an excessive amount of time.

  Reduce instances of the problem by remembering the last vnode which
  failed its timelock and using LK_NOWAIT for later pages.  A single
  vnode is remembered for now.

Reported-by: Francois Tigeot <ftigeot@wolfpond.org>