rust: cpumask: Mark CpumaskVar as transparent

Unsafe code in CpumaskVar's methods assumes that the type has the same
layout as `bindings::cpumask_var_t`. This is not guaranteed by
the default struct representation in Rust, but requires specifying the
`transparent` representation.

Fixes: 8961b8cb30 ("rust: cpumask: Add initial abstractions")
Signed-off-by: Baptiste Lepers <baptiste.lepers@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

rust/kernel/cpumask.rs

@@ -212,6 +212,7 @@ impl Cpumask {
 /// }
 /// assert_eq!(mask2.weight(), count);
 /// ```
+#[repr(transparent)]
 pub struct CpumaskVar {
     #[cfg(CONFIG_CPUMASK_OFFSTACK)]
     ptr: NonNull<Cpumask>,