rust: rbtree: simplify finding current in remove_current

The previous version used a verbose `match` to get
`current`, which may be slightly confusing at first
glance.

This change makes it shorter and more clearly expresses
the intent: prefer `next` if available, otherwise fall
back to `prev`.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708075850.25789-1-work@onurozkan.dev
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/rbtree.rs

@@ -775,23 +775,14 @@ impl<'a, K, V> Cursor<'a, K, V> {
         // the tree cannot change. By the tree invariant, all nodes are valid.
         unsafe { bindings::rb_erase(&mut (*this).links, addr_of_mut!(self.tree.root)) };
 
-        let current = match (prev, next) {
-            (_, Some(next)) => next,
-            (Some(prev), None) => prev,
-            (None, None) => {
-                return (None, node);
-            }
-        };
+        // INVARIANT:
+        // - `current` is a valid node in the [`RBTree`] pointed to by `self.tree`.
+        let cursor = next.or(prev).map(|current| Self {
+            current,
+            tree: self.tree,
+        });
 
-        (
-            // INVARIANT:
-            // - `current` is a valid node in the [`RBTree`] pointed to by `self.tree`.
-            Some(Self {
-                current,
-                tree: self.tree,
-            }),
-            node,
-        )
+        (cursor, node)
     }
 
     /// Remove the previous node, returning it if it exists.