hammer2 - Increase bulkfree buffer size request and cap

hammer2 - Increase bulkfree buffer size request and cap

* Increase the kernel buffer size cap for hammer2 cleanups
  (aka bulkfree's) from 64MB to 1/16 physical memory.  Systems
  with more memory can accomodate larger in-memory bulkfree bitmap
  buffers.  Systems with less memory simply do multiple scan
  passes as per usual.

* Increase the buffer size requested by the hammer2 utility
  from 8MB to 1/16 physical memory.  The actual amount of memory
  allocated by the kernel will be 32MB per 1TB of filesystem, up to
  the cap.

* This will generally reduce the number of scans required for
  a 4TB filesystem from 16 to 1, meaning that bulkfree will take
  1/16 the time as it did before for a filesystem of that size.

  For example, a system with 128GB of ram would be able to
  request up to 8GB of buffer which can accomodate a bulkfree
  operation on a 256TB filesystem in a single pass.