rust: maple_tree: add MapleTreeAlloc

rust: maple_tree: add MapleTreeAlloc

To support allocation trees, we introduce a new type MapleTreeAlloc for
the case where the tree is created using MT_FLAGS_ALLOC_RANGE.  To ensure
that you can only call mtree_alloc_range on an allocation tree, we
restrict thta method to the new MapleTreeAlloc type.  However, all methods
on MapleTree remain accessible to MapleTreeAlloc as allocation trees can
use the other methods without issues.

Link: https://lkml.kernel.org/r/20250902-maple-tree-v3-3-fb5c8958fb1e@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Andrew Ballance <andrewjballance@gmail.com>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>