rust: alloc: vec: Add dropped set_len() for ::drain()

To keep in sync with Rust's std::Vec::drain() implementation keep
set_len() around.

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

rust/kernel/alloc/kvec.rs

@@ -188,6 +188,19 @@ where
         self.len
     }
 
+    /// Forcefully sets `self.len` to `new_len`.
+    ///
+    /// # Safety
+    ///
+    /// - `new_len` must be less than or equal to [`Self::capacity`].
+    /// - If `new_len` is greater than `self.len`, all elements within the interval
+    ///   [`self.len`,`new_len`) must be initialized.
+    #[inline]
+    pub unsafe fn set_len(&mut self, new_len: usize) {
+        debug_assert!(new_len <= self.capacity());
+        self.len = new_len;
+    }
+
     /// Increments `self.len` by `additional`.
     ///
     /// # Safety