maple_tree: clean up mas_dfs_postorder()

maple_tree: clean up mas_dfs_postorder()

Convert loop type to ensure all variables are set to make the compiler
happy, and use the mas_is_none() function instead of explicitly checking
the node in the maple state.

Link: https://lkml.kernel.org/r/20230518145544.1722059-5-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: David Binderman <dcb314@hotmail.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>