rust: alloc: add Vec::truncate method

Implement the equivalent to the std's Vec::truncate on the kernel's Vec
type.

Link: https://lore.kernel.org/r/20250316111644.154602-2-andrewjballance@gmail.com
Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
[ Rewrote safety comment of set_len(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/alloc/kvec.rs

@@ -455,6 +455,41 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
 
         Ok(())
     }
+
+    /// Shortens the vector, setting the length to `len` and drops the removed values.
+    /// If `len` is greater than or equal to the current length, this does nothing.
+    ///
+    /// This has no effect on the capacity and will not allocate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    /// v.truncate(1);
+    /// assert_eq!(v.len(), 1);
+    /// assert_eq!(&v, &[1]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn truncate(&mut self, len: usize) {
+        if len >= self.len() {
+            return;
+        }
+
+        let drop_range = len..self.len();
+
+        // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
+        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
+
+        // SAFETY: By the above bounds check, it is guaranteed that `len < self.capacity()`.
+        unsafe { self.set_len(len) };
+
+        // SAFETY:
+        // - the dropped values are valid `T`s by the type invariant
+        // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
+        //   len, therefore we have exclusive access to [`new_len`, `old_len`)
+        unsafe { ptr::drop_in_place(ptr) };
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {