maple_tree: simplify mas_is_span_wr()

Make the code for detecting spanning writes more concise.

Link: https://lkml.kernel.org/r/20230524031247.65949-5-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>