kernel - repurpose buffer cache entries under heavy I/O loads

kernel - repurpose buffer cache entries under heavy I/O loads

* At buffer-cache I/O loads > 200 MBytes/sec (newbuf instantiations, not
  cached buffer use), the buffer cache will now attempt to repurpose the
  VM pages in the buffer it is recycling instead of returning the pages
  to the VM system.

* sysctl vfs.repurposedspace may be used to adjust the I/O load limit.

* The repurposing code attempts to free the VM page then reassign it to
  the logical offset and vnode of the new buffer.  If this succeeds, the
  new buffer can be returned to the caller without having to run any
  SMP tlb operations.  If it fails, the pages will be either freed or
  returned to the VM system and the buffer cache will act as before.

* The I/O load limit has a secondary beneficial effect which is to reduce
  the allocation load on the VM system to something the pageout daemon can
  handle while still allowing new pages up to the I/O load limit to transfer
  to VM backing store.  Thus, this mechanism ONLY effects systems with I/O
  load limits above 200 MBytes/sec (or whatever programmed value you decide
  on).

* Pages already in the VM page cache do not count towards the I/O load limit
  when reconstituting a buffer.