rust: cpumask: Replace MaybeUninit and mem::zeroed with Opaque APIs

Replace the following unsafe initializations:
1. `MaybeUninit::uninit().assume_init()` with `Opaque::uninit()`
2. `core::mem::zeroed()` with `Opaque::zeroed()`

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: Rust-for-Linux#1178
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/CAH5fLgj0OoCn56OkNUmiPQ=RAVa_VmS-yMZ4TNBSpGPNtZ5D0A@mail.gmail.com/
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Ritvik Gupta <ritvikfoss@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Author: ritvikos

rust/kernel/cpumask.rs

@@ -14,9 +14,6 @@ use crate::{
 #[cfg(CONFIG_CPUMASK_OFFSTACK)]
 use core::ptr::{self, NonNull};
 
-#[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-use core::mem::MaybeUninit;
-
 use core::ops::{Deref, DerefMut};
 
 /// A CPU Mask.
@@ -239,10 +236,7 @@ impl CpumaskVar {
             },
 
             #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-            // SAFETY: FFI type is valid to be zero-initialized.
-            //
-            // INVARIANT: The associated memory is freed when the `CpumaskVar` goes out of scope.
-            mask: unsafe { core::mem::zeroed() },
+            mask: Cpumask(Opaque::zeroed()),
         })
     }
 
@@ -266,10 +260,7 @@ impl CpumaskVar {
                 NonNull::new(ptr.cast()).ok_or(AllocError)?
             },
             #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-            // SAFETY: Guaranteed by the safety requirements of the function.
-            //
-            // INVARIANT: The associated memory is freed when the `CpumaskVar` goes out of scope.
-            mask: unsafe { MaybeUninit::uninit().assume_init() },
+            mask: Cpumask(Opaque::uninit()),
         })
     }