maple_tree: fix depth tracking in maple_state

It is possible to confuse the depth tracking in the maple state by
searching the same node for values.  Fix the depth tracking by moving
where the depth is incremented closer to where the node changes level. 
Also change the initial depth setting when using the root node.

Link: https://lkml.kernel.org/r/20221107163814.866612-1-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>