kernel - Fix MADV_NOSYNC and MAP_NOSYNC, improve vkernel performance

kernel - Fix MADV_NOSYNC and MAP_NOSYNC, improve vkernel performance

* The vm_prefault() code was not setting PG_NOSYNC so only 1/4 of the
  pages of a NOSYNC memory mapping were actually NOSYNC.

* This bug caused the vkernel to essentially flush out all of its
  dirty memory pages every 30 seconds.  Needless to say this was bad.

  The vkernel can now be run with its memory set in the multiples
  of gigabytes (if you happen to have that much real memory) without
  creating a massive disk load.