maple_tree: remove pointer to pointer use in mas_alloc_nodes()

There is a more direct and cleaner way of implementing the same functional
code.  Remove the confusing and unnecessary use of pointers here.

Link: https://lkml.kernel.org/r/20221026151241.4031117-1-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>