maple_tree: use macro MA_ROOT_PARENT instead of number

When you need to compare whether node->parent is parent of the
root node, using macro MA_ROOT_PARENT is easier to understand
and for better readability.

Link: https://lkml.kernel.org/r/20221221060058.609003-5-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>