maple_tree: Rework node split calculation for better non-leaf node split

maple_tree: Rework node split calculation for better non-leaf node split
usage.

When calculating where to split non-leaf nodes, assume the minimum span
is 8 in each entry, so 8*8.  This still needs work as we could figure
out the level from a leaf and keep going up in the minimum span to
better utilize more levels of the tree.

There is also the issue of the right-most entry (ULONG_MAX) which ends
up wasting a slot in the linear allocation case.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>