kernel - Make VM fault waits in low memory the same as other low memory waits

kernel - Make VM fault waits in low memory the same as other low memory waits

* Nominal memory allocations, particularly by tmpfs, allow allocations
  down to the 'min' before waiting for memory to recover to 'target'.
  The VM fault code required memory to be at or above the 'target'
  at all times.  This led to extreme starvation because other user-accessible
  subsystems could force free memory to remain below 'target'.

* Change the VM code to use substantially the same down-to 'min' and
  recover-to 'target' parameters that other subsystems use.  This
  should reduce the excessive stalls under low memory conditions.

* Rename vm_waitpfault() to vm_wait_pfault() to conform with other
  naming schemes in the same API.