rust: alloc: add missing invariant in Vec::set_len()

When setting a new length, we have to justify that the set length
represents the exact number of elements stored in the vector.

Reviewed-by: Benno Lossin <[email protected]>
Reported-by: Alice Ryhl <[email protected]>
Closes: https://lore.kernel.org/rust-for-linux/[email protected]
Fixes: 2aac4cd ("rust: alloc: implement kernel `Vec` type")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Danilo Krummrich <[email protected]>

Author: Danilo Krummrich

rust/kernel/alloc/kvec.rs

@@ -193,6 +193,9 @@ where
     #[inline]
     pub unsafe fn set_len(&mut self, new_len: usize) {
         debug_assert!(new_len <= self.capacity());
+
+        // INVARIANT: By the safety requirements of this method `new_len` represents the exact
+        // number of elements stored within `self`.
         self.len = new_len;
     }