maple_tree: use height and depth consistently

maple_tree: use height and depth consistently

For the maple tree, the root node is defined to have a depth of 0 with a
height of 1. Each level down from the node, these values are incremented
by 1. Various code paths define a root with depth 1 which is inconsisent
with the definition. Modify the code to be consistent with this
definition.

Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>