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.