kernel - Add vfs.repurpose_enable, adjust B_HASBOGUS

kernel - Add vfs.repurpose_enable, adjust B_HASBOGUS

* Add vfs.repurpose_enable, default disabled.  If this feature is turned on
  the system will try to repurpose the VM pages underlying a buffer on
  re-use instead of allowing the VM pages to cycle into the VM page cache.
  Designed for high I/O-load environments.

* Use the B_HASBOGUS flag to determine if a pmap_qenter() is required,
  and devolve the case to a single call to pmap_qenter() instead of one
  for each bogus page.