kernel - refactor vm_page busy

kernel - refactor vm_page busy

* Move PG_BUSY, PG_WANTED, PG_SBUSY, and PG_SWAPINPROG out of m->flags.

* Add m->busy_count with PBUSY_LOCKED, PBUSY_WANTED, PBUSY_SWAPINPROG,
  and PBUSY_MASK (for the soft-busy count).

* Add support for acquiring a soft-busy count without a hard-busy.
  This requires that there not already be a hard-busy.  The purpose
  of this is to allow a vm_page to be 'locked' in a shared manner
  via the soft-busy for situations where we only intend to read from
  it.