maple_tree: Add support for splitting nodes at the last level.

maple_tree: Add support for splitting nodes at the last level.

It is possible that we run out of room in the node that is targeted for
the insert.  It is not possible to know if there is room or not as the
insert may require one or two inserts, so split once it is known.

Also change where the split occurs based on the range size.

Add some more tests including a huge number of nodes to be added.

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