maple_tree: add mas_wr_new_end() to calculate new_end accurately

The previous new_end calculation is inaccurate, because it assumes that
two new pivots must be added (this is inaccurate), and sometimes it will
miss the fast path and enter the slow path.  Add mas_wr_new_end() to
accurately calculate new_end to make the conditions for entering the fast
path more accurate.

Link: https://lkml.kernel.org/r/20230524031247.65949-7-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>