rust: maple_tree: add __rust_helper to helpers

This is needed to inline these helpers into Rust code.

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Andrew Ballance <andrewjballance@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Link: https://patch.msgid.link/20260105-define-rust-helper-v2-10-51da5f454a67@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/helpers/maple_tree.c

@@ -2,7 +2,8 @@
 
 #include <linux/maple_tree.h>
 
-void rust_helper_mt_init_flags(struct maple_tree *mt, unsigned int flags)
+__rust_helper void rust_helper_mt_init_flags(struct maple_tree *mt,
+					     unsigned int flags)
 {
 	mt_init_flags(mt, flags);
 }