hammer2 - Reformulate strategy code and direct I/O

hammer2 - Reformulate strategy code and direct I/O

* Use variable block sizes for both logical and physical buffers such
  that the logical buffer matches the physical buffer.

* Change the file data layout to use 64K blocks for all bulk data fully
  enclosed in the block, with a single variable-length block straddling
  the file EOF sized 1K - 64K in powers of 2.  The inode's 512 bytes of
  embedded data is still implemented for file sizes <= 512 bytes.

* Implement direct IO for both reading and writing.

* Reformulate the strategy write code such that bio2.bio_offset is always
  pre-calculated, allowing the strategy code to issue the I/O without
  requiring any further filesystem interactions.

* Fixes numerous deadlocks.