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: 8961b8cb ("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>