rust: rbtree: add immutable cursor

Sometimes we may need to iterate over, or find an element in a read
only (or read mostly) red-black tree, and in that case we don't need a
mutable reference to the tree, which we'll however have to take to be
able to use the current (mutable) cursor implementation.

This patch adds a simple immutable cursor implementation to RBTree,
which enables us to use an immutable tree reference. The existing
(fully featured) cursor implementation is renamed to CursorMut,
while retaining its functionality.

The only existing user of the [mutable] cursor for RBTrees (binder) is
updated to match the changes.

Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.se>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251014123339.2492210-1-vitaly.wool@konsulko.se
[ Applied `rustfmt`. Added intra-doc link. Fixed unclosed example.
  Fixed docs description. Fixed typo and other formatting nits.
    - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Vitaly Wool

drivers/android/binder/freeze.rs

@@ -321,7 +321,7 @@ impl Process {
             KVVec::with_capacity(8, GFP_KERNEL).unwrap_or_else(|_err| KVVec::new());
 
         let mut inner = self.lock_with_nodes();
-        let mut curr = inner.nodes.cursor_front();
+        let mut curr = inner.nodes.cursor_front_mut();
         while let Some(cursor) = curr {
             let (key, node) = cursor.current();
             let key = *key;
@@ -335,7 +335,7 @@ impl Process {
                 // Find the node we were looking at and try again. If the set of nodes was changed,
                 // then just proceed to the next node. This is ok because we don't guarantee the
                 // inclusion of nodes that are added or removed in parallel with this operation.
-                curr = inner.nodes.cursor_lower_bound(&key);
+                curr = inner.nodes.cursor_lower_bound_mut(&key);
                 continue;
             }
 

drivers/android/binder/process.rs

@@ -1293,7 +1293,7 @@ impl Process {
         {
             while let Some(node) = {
                 let mut lock = self.inner.lock();
-                lock.nodes.cursor_front().map(|c| c.remove_current().1)
+                lock.nodes.cursor_front_mut().map(|c| c.remove_current().1)
             } {
                 node.to_key_value().1.release();
             }

drivers/android/binder/range_alloc/tree.rs

@@ -207,7 +207,7 @@ impl<T> TreeRangeAllocator<T> {
     }
 
     pub(crate) fn reservation_abort(&mut self, offset: usize) -> Result<FreedRange> {
-        let mut cursor = self.tree.cursor_lower_bound(&offset).ok_or_else(|| {
+        let mut cursor = self.tree.cursor_lower_bound_mut(&offset).ok_or_else(|| {
             pr_warn!(
                 "EINVAL from range_alloc.reservation_abort - offset: {}",
                 offset