rust: Use CpuId in place of raw CPU numbers

Use the newly defined `CpuId` abstraction instead of raw CPU numbers.

This also fixes a doctest failure for configurations where `nr_cpu_ids <
4`.

The C `cpumask_{set|clear}_cpu()` APIs emit a warning when given an
invalid CPU number — but only if `CONFIG_DEBUG_PER_CPU_MAPS=y` is set.

Meanwhile, `cpumask_weight()` only considers CPUs up to `nr_cpu_ids`,
which can cause inconsistencies: a CPU number greater than `nr_cpu_ids`
may be set in the mask, yet the weight calculation won't reflect it.

This leads to doctest failures when `nr_cpu_ids < 4`, as the test tries
to set CPUs 2 and 3:

  rust_doctest_kernel_cpumask_rs_0.location: rust/kernel/cpumask.rs:180
  rust_doctest_kernel_cpumask_rs_0: ASSERTION FAILED at rust/kernel/cpumask.rs:190

Fixes: 8961b8cb ("rust: cpumask: Add initial abstractions")
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72k3ozKkLMinTLQwvkyg9K=BeRxs1oYZSKhJHY-veEyZdg@mail.gmail.com/


Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
Closes: https://lore.kernel.org/all/87qzzy3ric.fsf@kernel.org/


Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>