maple_tree: use mt_node_max() instead of direct operations mt_max[]

Use mt_node_max() to get the maximum number of slots for a node,
rather than direct operations mt_max[], makes it better portability.

Link: https://lkml.kernel.org/r/20221221060058.609003-4-vernon2gm@gmail.com


Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>