maple_tree: relocate the declaration of mas_empty_area_rev().

Relocate the declaration of mas_empty_area_rev() so that mas_empty_area()
and mas_empty_area_rev() are together.

Link: https://lkml.kernel.org/r/20230524031247.65949-11-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>