maple_tree: mark three functions as __maybe_unused

People keep trying to remove three functions that are going to be used in
a feature that is being developed.  Dropping the functions entirely may
end up with people trying to use the bit for other uses, as people have
tried in the past.

Adding __maybe_unused stops compilers complaining about the unused
functions so they can be silently optimised out of the compiled code and
people won't try to claim the bit for another use.

Link: https://lore.kernel.org/all/20230726080916.17454-2-zhangpeng.00@bytedance.com/
Link: https://lore.kernel.org/all/202408310728.S7EE59BN-lkp@intel.com/
Link: https://lkml.kernel.org/r/20240907021506.4018676-1-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>