FROMGIT: rust: alloc: add Vec::resize method

Implement the equivalent of the rust std's Vec::resize on the kernel's
Vec type.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250316111644.154602-3-andrewjballance@gmail.com
Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
[ Use checked_sub(), as suggested by Tamir. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Bug: 414994413
(cherry picked from commit 1679b7159379d11100e4ab7d1de23c8cd7765aa1
 https://github.com/Rust-for-Linux/linux.git alloc-next)
Change-Id: I34b82e880ca6335038461be0de38fbb1a6854cba
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/kernel/alloc/kvec.rs

@@ -556,6 +556,33 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
 
         Ok(v)
     }
+
+    /// Resizes the [`Vec`] so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is smaller than `len`, the `Vec` is [`Vec::truncate`]d.
+    /// If `new_len` is larger, each new slot is filled with clones of `value`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    /// v.resize(1, 42, GFP_KERNEL)?;
+    /// assert_eq!(&v, &[1]);
+    ///
+    /// v.resize(3, 42, GFP_KERNEL)?;
+    /// assert_eq!(&v, &[1, 42, 42]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
+        match new_len.checked_sub(self.len()) {
+            Some(n) => self.extend_with(n, value, flags),
+            None => {
+                self.truncate(new_len);
+                Ok(())
+            }
+        }
+    }
 }
 
 impl<T, A> Drop for Vec<T, A>