maple_tree: make the code symmetrical in mas_wr_extend_null()

Just make the code symmetrical to improve readability.

Link: https://lkml.kernel.org/r/20230524031247.65949-6-zhangpeng.00@bytedance.com


Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>