rust: cpumask: rename methods of Cpumask for clarity and consistency

rust: cpumask: rename methods of Cpumask for clarity and consistency

Rename `as_ref` and `as_mut_ref` to `from_raw` and `from_raw_mut` to
align with the established naming convention for constructing types
from raw pointers in the kernel's Rust codebase.

Signed-off-by: Yilin Chen <1479826151@qq.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>