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.comSigned-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>

lib/maple_tree.c

@@ -348,17 +348,17 @@ static inline void *mte_safe_root(const struct maple_enode *node)
 	return (void *)((unsigned long)node & ~MAPLE_ROOT_NODE);
 }
 
-static inline void *mte_set_full(const struct maple_enode *node)
+static inline void __maybe_unused *mte_set_full(const struct maple_enode *node)
 {
 	return (void *)((unsigned long)node & ~MAPLE_ENODE_NULL);
 }
 
-static inline void *mte_clear_full(const struct maple_enode *node)
+static inline void __maybe_unused *mte_clear_full(const struct maple_enode *node)
 {
 	return (void *)((unsigned long)node | MAPLE_ENODE_NULL);
 }
 
-static inline bool mte_has_null(const struct maple_enode *node)
+static inline bool __maybe_unused mte_has_null(const struct maple_enode *node)
 {
 	return (unsigned long)node & MAPLE_ENODE_NULL;
 }