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.