maple_tree: remove mas_searchable()

Now that the status of the maple state is outside of the node, the
mas_searchable() function can be dropped for easier open-coding of what is
going on.

Link: https://lkml.kernel.org/r/20231101171629.3612299-10-Liam.Howlett@oracle.com


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