HAMMER VFS - Improve initial B-Tree packing

HAMMER VFS - Improve initial B-Tree packing

* Detect the case where B-Tree leafs are being laid down sequentially,
  such as when creating a large file.  When linear operation is detected
  split leafs 75:25 instead of 50:50.  This greatly improves fill ratios.

  It should be noted that the HAMMER flush sorts by inode so directory
  entries will also tend to benefit.

* This only effects (improves) the initial B-Tree layout.  The overnight
  hammer cleanup will refactor the B-Tree to a more optimal state
  regardless.